Light source device and optical communication module employing the device

ABSTRACT

A light source device for radiating a stimulated emission from a semiconductor laser to outside via a multiple scattering optical system, which system has a first region located adjacent to the semiconductor laser and a second region that abuts on the first region and reaches the outside. The first region contains scatterers at a higher density than the second region does. The light source device has an amount of near-field pattern speckles σ PAR  of 3×10 −3  or more. The second region may have a lens portion as a magnifier for at least a principle part of a secondary planar light source formed at an interface between the first and second regions.

This application is the US national phase of international applicationPCT/JP03/02418 filed 3 Mar. 2003, which designated the US and claimspriority to JP Application No. 2002-63942 filed 8 Mar. 2002. The entirecontents of these applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a light source device resin-sealed bytransfer molding, potting or the like so that the device is providedwith an optical lens function, and to an optical communication modulethat employs the device.

BACKGROUND ART

In recent years, infrared communication modules conforming to the IrDA(Infrared Data Association) standard have been rapidly scaled down insize, and the product of a short distance (20 cm) specification has itslens portion diameter and height reduced to about 1 mm to 2 mm.Concerning the communication speed, there is a gradual increase in speedwith regard to IrDA, while compatibilities of a communication distanceof several meters with high-speed characteristics of up to about 100Mbps between a base station where a sufficient quantity of light isprovided by arranging parallel twenty or more bullet type LED's and aterminal unit that has a sharp directivity and a tracking function arebeing established in optical wireless LAN products.

Such the wireless optical communication technique, which also has aproblem of directivity and shielding, is expected to develop itsapplications as a high-speed interface of a palmtop or hand-held typeportable terminal unit taking advantage of its high-speedcharacteristic, secrecy and particularly advantages in terms of cost.However, the high-speed optical wireless LAN (Local Area Network)products generally have a large size and considerably large powerconsumption. Moreover, there have been not a few attempts to use acomparatively inexpensive semiconductor laser of a near infraredwavelength region for wireless optical communications by making thesemiconductor laser eye safe attaching importance to the high-speedperformance in the past. However, there have been used comparativelylarge-scale diffusers and beam-shaping optical systems, and it has beendifficult to achieve scaledown in size and low cost equivalent to thoseof the products conforming to IrDA.

That is, there has not yet been put to practical use a small-sizedinexpensive optical communication module necessary for achieving awireless optical communication system that buries the gap between thetwo of the existing IrDA and the optical wireless LAN and has ahigher-speed wider-range communication area.

DISCLOSURE OF THE INVENTION

Accordingly, an object of this invention is to provide a light sourcedevice which, with a simple construction, is capable of obtaining a highlight output efficiency while securing eye safety even if a high-powersemiconductor light-emitting device is employed, and allowing reductionin power consumption, size and cost to be achieved and suitable for ahigh-speed optical communication system that covers a wide communicationarea, and to provide an optical communication module that employs thedevice.

For the last few years there has been a significant progress in thetechnology of increasing the output of the GaAs-based semiconductorlaser of the near infrared region (780-nm band and 980-nm band) used forhigh-speed CD-R/RW drives and fiber amplifier excitation light sources.At present, there have been achieved a high power on the level of 100 mWto 300 mW of CW (Continuous Wave) and reliability of not shorter thanseveral thousands of hours in single basic transverse mode operationwith a narrow stripe width of about 2 μm to 6 μm. Moreover, anInGaN-based semiconductor laser, which has an emission wavelength in theblue or ultraviolet region, has also been developed for practical use,and a high power of not smaller than 30 mW usable as a light source forwriting the next-generation optical discs has become able to be stablyobtained.

In an application in which a beam is emitted into a free space, the eyesafety prescribed by an international safety standard IEC60825-1 and aJapanese standard JIS C6082 and so on must be satisfied. Particularly,in the applications to consumer equipment, there is demanded the Class 1level eye safety according to which no risk of depriving a human beingof his or her eyesight is caused even when the output light from a lightsource directly enters a human eye via some optical system.

Accordingly, if the eye safe technology for converting the output lightof a high-power semiconductor laser like the aforementioned one withoutloss to a specific apparent light source size is established, then a keydevice for the next-generation wireless optical communication aspreviously described is achieved. The present applicant paid attentionto the technology of reducing the spatial coherency bythree-dimensionally introducing a scattering material into a resinsealed module or a transmission lens as an elemental technology for thepurpose.

In the field of a light-emitting diode (LED) that is an incoherent lightsource, it is well-known that various powder materials (so-calledfillers) are mixed in the sealing resin according to the purpose ofimproving the luminance unevenness, wavelength conversion for a whiteLED and so on. Good examples can be found in, for example, JP 59-112665A and JP 2000-200928 A. However, there exists no example in which thescattering material (filler) technology is examined in detail from theviewpoint of reducing the high spatial coherency in a small-sized lightsource device equipped with a semiconductor laser so long as the presentapplicant knows.

On the other hand, in the fields of coherent optics and lighting optics,the fact that an identical radiant intensity distribution can beconstituted of an arbitrary planar light source having a varied spatialdistribution of the degree of coherence is known as “Equivalence Theoremof Planar Source” (refer to, for example, Mandel & Wolf, OpticalCoherence and Quantum Optics, 1995, Cambridge University Press, Chapter5). That is, it is theoretically possible to constitute an opticalsystem so that the angular distribution of the radiant intensity is notdifferent from that of a semiconductor laser by diffusing the outputlight of the semiconductor laser through some scatterer.

However, it is general to change locally and at random the phase and thescattering angle of incident light of scatterers utilized inapplications or implementations in the aforementioned field by utilizingthe roughness and configuration of the surface or interface asrepresented by a diffuser made of a frosted glass or a hologram relief.These are optical elements independent of the light source element andneed a separate fixation means and a space therefor. Therefore, theentire light source cannot help becoming bulky (increased in volume).Otherwise, although the techniques of rotating the diffuser around theaxis and forming a dynamic diffraction grating by applying ultrasonicwaves to a liquid or the like are also utilized, the optical systemnaturally comes to have a larger scale.

Moreover, when the properties of an optically thick sample tightlystuffed with scatterers such as a biological tissue is analyzed, aforward scattering characteristic is generally examined with atransparent arrangement. Researches on the characteristics of theattenuation of coherent scattering light component, depolarization andso on of individual samples of various types of particles, bloods,tissues and so on as fundamental data have been accumulated. However,there have been scarcely conducted researches for systematicallyexpressing a method for constituting a multiple scattering opticalsystem from the viewpoint of making the coherency of the laser lightsource effectively disappear in a minute volume as desired in thisinvention.

There has conventionally been an attempt to reduce the cost by adoptinga resin mold package instead of a hermetically sealed CAN package thatis the mounting form of an ordinary semiconductor laser. Referenceshould be made to, for example, JP No. 2927044 and JP 08-236873 A.However, these technologies are based on a major premise that the highspatial coherency originally possessed by the semiconductor laser isretained for applications to the image formation optical systems ofoptical discs and so on, and the purport, applications and technicalbackgrounds of them largely differ from those of the multiple scatteringoptical system to which this invention is directed.

This application discloses various methods for actually manufacturingand putting into practice a small-sized light source device and acomponent module, which are equipped with a high-power semiconductorlaser and able to emit the output light safely to the outside asdescribed hereinabove. According to the construction disclosed in thisapplication, in a module that has an apparent light source size of adiameter of, for example, 1 mm and of which the spatial coherency issufficiently reduced, there is formed a beam controlled so as to emanateto a free space at a full-width-at-half-maximum radiation angle of 30°.In this case, conforming to the international safety standard IEC80625-1 Amendment 2, an optical output of a maximum of about 150 mW atCW is permitted in, for example, a wavelength band of 850 nm. Inpractice, a light source for a wireless optical communication modulewould be conceivably a high-efficiency subminiature light source whichsatisfies specification requirements of Class 1 level eye safety, aradiation angle that is wide to some extent and is not too wide (e.g.,60°≧ full-width-at-half-maximum radiation angle ≧30°), restrainedunnecessary wide-angle radiation (skirt trailing), a peak output of 120mW at a drive current of not greater than 140 mA, and a module totalthickness of not greater than 3 mm.

Accordingly, the present inventors examined in detail a method for usinga multiple scattering optical system in which scatterers were dispersedat a high density in a specified region inside a structure in order toconstitute a light source device that satisfactorily controlled theradiant intensity distribution while reducing the coherency of theoutput light from a semiconductor laser inside an extremely minutethree-dimensional structure that typically had a dimension of notgreater than about one millimeter to several millimeters typically inthe axis direction of the output light. Particularly, with regard to thecorrelation between an optical depth or a mean free path and transportfree path in the minute three-dimensional structure depending on thediameter, refractive index and dispersion density of the scatterers, andspeckles generated as a consequence of random interference process andfurther with regard to the upper limit and the lower limit of apracticable dispersion density range, detailed examinations wererepetitively carried out to find the conditions that satisfy the Class 1eye safety. Moreover, comparison to the scattered light optical systemusing a diffuser and comparison to the case where an LED, an SLD(Super-Luminescent Diode) of a low coherency, or a broad areasemiconductor laser is used as a light source device were systematicallycarried out.

As a result, it was found that the speckles were extremely effectivelyreduced and compatibilities with the other optical characteristics couldbe established by using various evaluation methods and constituent meansof the optical system disclosed in this invention although a specklepattern on a serious level could be generated with regard to the eyesafety and the uniformity of radiant intensity in the near-field patternand the far-field pattern in the minute multiple scattering opticalsystem that employed a high-power semiconductor laser provided with thenarrow-stripe optical waveguide structure. Conversely, the constituentmeans of the multiple scattering optical system can also be preferablyapplied to a light source element other than the high-powersemiconductor laser. For example, there can be enumerated a broad areasemiconductor laser, a surface emitting laser (VCSEL: Vertical CavitySurface Emitting Laser) and their array bars, two-dimensional arrays,phased arrays and so on. Furthermore, the constituent means of themultiple scattering optical system can restrain the optical loss to anextremely small level, and this arrangement is therefore effective alsoin achieving eye safe by enlarging the apparent light source size of thelight source element that is a light source element of a comparativelylow timewise coherency like SLD and has a light emission spot smallerthan that of an ordinary LED (e.g., several hundreds of micrometerssquare) with an optical waveguide structure.

If a semiconductor laser of a material system capable of oscillating inthe so-called intermediate or far infrared region or a quantum cascadestructure is employed, it is possible to secure eye safety even ifintense near-field pattern speckles remain. However, for communicationuse, speckles particularly in the far-field pattern possibly becomepractically a serious problem. Therefore, it is required to sufficientlyreduce the speckles of both the far-field pattern and the near-fieldpattern. Although the far-field pattern speckles also tend to befundamentally reduced by taking measures for reducing the near-fieldpattern speckles, the speckles of the far-field pattern might besaturated on the level of some improvement. However, in the case whereeye safety is secured, the condition can be regarded practicallyallowable unless a disturbance (speckle amplitude fluctuation) thatfalls below the halfpower occurs at an angle within a range of thehalf-value angle (radiation angle at which the halfpower of peakintensity is achieved) of the radiant intensity.

With regard to the speckles of the near-field pattern, an optical systemmade cloudy with the scattering material, which at first glance seems tohave a low light-concentrating property at a glance and provokes afeeling of security, might therefore rather jeopardize the human body.Moreover, even in the case where a diffuser or a hologram opticalelement designed comparatively smartly is employed, it is not easy tomake the speckles completely disappear.

For reference, FIG. 16 shows the construction of an optical system inwhich the spatial coherency of a semiconductor laser is reduced byemploying so-called the diffuser seen in JP 08-264885 A. As shown inFIG. 16, a semiconductor laser 1600 is fixed on a stem 1601 by diebonding, and electrical connection is achieved by wire bonding. Then, acap 1602 that covers the stem 1601 equipped with the semiconductor laser1600 is provided with a sealing member 1603 that serves as a diffuser inplace of an ordinary low-reflectance coated glass. One surface of thesealing member 1603 is formed into an appropriate roughened surface toreduce the spatial coherency of outgoing light from the semiconductorlaser 1600. The diffuser is thinly formed by using the etching andhologram technologies for a transparent semiconductor substrate,constituting a small-sized light source device. The near-field patternand the far-field pattern of this light source device were evaluated bya method as described later, and there was some case where the specklesof the far-field pattern can be produced within a permissible range.However, a wide-angle component greater than the half-value angle waslarge in the radiant intensity distribution (FFP), and the radiantintensity on the axis was reduced by 20 percent or more at maximum withrespect to the FFP of an ideal Gaussian configuration. Furthermore, itwas discovered that extremely large speckles were generated in thenear-field pattern and satisfying the Class 1 eye safety was difficulteven when the scattering plane of the diffuser was improved.

According to the construction of FIG. 16, the stimulated emission fromthe semiconductor laser 1600 reaches the scattering plane of the sealingmember 1603 via a sealing gas region 1604 where the light is neitherabsorbed nor scattered and is radiated as scattered light 1605 into afree space. When the diffuser of which the surface is roughened isemployed, the scattering frequency that the laser beam undergoes isextremely few and typically about one to a few times at most. Therefore,the spatial coherency is not lost at least in a local region on thelevel of the particle size of the roughened surface of the sealingmember 1603, and remarkable speckles are generated in the near-fieldpattern by interactions inside the particles or between adjoiningparticles. Moreover, when the diffuser of which the surface is roughenedis employed, the apparent light source size is geometrically determinedby the radiation angle characteristic of the laser element and thedistance to the scattering plane. Therefore, it becomes practicallydifficult to enlarge the size to the desired size in a small-sized lightsource device of which the dimensions of portions are prefixed.Conversely, if it is tried to obtain a large light source size, thedistance between the laser element and the scattering plane cannot helpbeing made large.

If the output light from a minute light source that has such near-fieldpattern speckles is incident on an eyeball via (or not via, depending onthe situation) some optical system, then the concentration of localpower or energy density is incurred by the minute structure of thespeckles included in the image focused on the retina, possibly causingretina thermal injury peculiar to the laser light source. Therefore, itis desired to scale down the size of the structure of the specklepattern to a level lower than the natural movement of the eyeball in afixation state and reduce the timewise and spatial coherency until thedisturbance amplitude becomes unable to be obviously observed.

In general, the human eyeball is involuntarily moving without stoppingeven when staring at one point. Three components of flick (one leap inan interval of about 0.03 seconds to 5 seconds at an angle of about 20minutes), drift (drift at about 0 to 30 minutes per second) and tremor(tremor at about 15 minutes at 30 Hz to 100 Hz) are generally called theflicks. The position of the image on the retina is quivering severaltens of times per second with an amplitude of about several tens ofmicrometers as a high-frequency component, and the quantity of thismovement agrees with the fact that recognizing the contrast of parallelstripes of several tens of lines or more per millimeter in terms ofspatial frequency is generally difficult.

In the light intensity spatial distribution of the typically observedspeckles in a perilous state, minute structures of a sharp fall to orbelow 1/e (e: base of natural logarithm) of the maximum value thereofare spatially two-dimensionally distributed at random. Particularly,when numbers of minute peak spots that have a span equivalent to theamount of movement (not smaller than about 10 μm) of the image on theretina due to the vibration components of about 100 Hz or up to about0.1 mm are contained in the near-field pattern, the energy density perunit area at each spot becomes increased to, for example, five or moretimes the average value, possibly exerting serious influence on theretina.

Under the situation in which the light source size is enlarged by takingsome measures for reducing the spatial coherency against the laser lightsource, an essential difference concerning eye safety from that of LEDis narrowed down to the above-mentioned point. When a beam from acertain light source is concentrated on the retina via some lightconcentrating optical system, the image spot size is strongly influencedby the chromatic aberration of the light concentrating system. Actually,the reason why light having a wide continuous spectrum range of LED orthe like cannot be concentrated on the neighborhood of the wavelengthlimit is largely ascribed to the influence of the chromatic aberrationas well as low spatial coherency. Although the latter is not consideredby the current safety standard and is hard to formulate, the means andeffects of this invention, which is mainly intended for a laser lightsource, suffer no alteration. It is a matter of course that theinvention can also be suitably used for enlarging the light-emittingspot of an LED where no speckle occurs and a light source element (e.g.,SLD) that has another optical waveguide structure without loss.

The situation, in which the near-field pattern of a single plane lightsource includes speckles as described above, differs from aconsideration for an array light source by IEC60825-1 standard or thelike in the following points. That is, the optical power included ineach individual speckle spot shares only a small rate to the totaloutput from the laser or the optical power possessed by the entireplanar light source. An increment in the local energy density or powerdensity at each spot is expressed by the ratio of a local maximum valueof PAR (Peak-to-Average Ratio) to the peak spot area. Therefore, if areduction in each individual speckle spot size to or below the level ofthe aforementioned tremor and a reduction in the amplitude offluctuation from the expected value (=1) of PAR typically to or belowthe level of one hundredth are made compatible, it is possible to securethe Class 1 level eye safety equivalent to that of an LED light sourcethat has the same apparent size.

The features of the objective minute multiple scattering optical systemof this invention and the problems to be solved are listed as follows.

First of all, a first feature is that light incident on the multiplescattering optical system can be regarded almost as a beam emitted froma point light source. With regard to the entire light source device,there is supposed the construction of an integrated body including alight source and also its exterior as a microoptical system formed on aresin substrate, a lead frame or a stem. Therefore, it is not permittedto carry out optical operation for reducing the spatial coherency so asto make a beam incident on a multiple scattering region after the beamdiameter is expanded to or larger than several millimeters by means of abeam expander, as generally carried out in an ordinary coherent opticalsystem.

As a second feature, an extremely small optical system of which thegeometrical distance from the light source to the exterior is about onemillimeter to several millimeters in a multiple scattering opticalsystem is thought of. Therefore, it is not permitted to constitute anoptical system arbitrarily elongated in the optical axis direction inorder to increase the scattering frequency. Moreover, it is difficult toadopt a construction that has difficulties or impossibilities in theformation of integrated components such as movable sections andcomposite lens systems. In addition, the fact that the accuracies ofprocesses and constituent elements tend to be insufficient due to thesmall size of the entire optical system also adds difficulties to thesolution of the problems.

As a third feature, the light source element to be employed shouldpreferably be a high-power semiconductor laser of a single transversemode having a comparatively narrow stripe width of about 1 μm to 10 μm.By employing such a light source, a remarkable reduction in the currentconsumption and a high power that has conventionally been impracticablecan be made compatible with high speed in comparison with the case wherea light source of LED, SLD or the like of a low coherency having acontinuous wavelength spectrum distribution is employed.

However, in the case where the ordinary narrow-stripe semiconductorlaser operates at or above the milliwatt level, the spectral linewidthis typically about 10 MHz, and the coherence length (maximum opticalpath difference interferable by two light waves divided in amplitude) isabout several tens of meters. During high-power operation of not smallerthan several tens of milliwatts, the line width is generally narrowed inproportion to the reciprocal of the output. However, according tocircumstances, it is sometimes the case where the spectral linewidth isextended to or greater than about 10 MHz and the coherence lengthbecomes equal to or smaller than about several meters due to the effectsof the nonlinearity of hole burning (local gain saturation due to thenonuniformity of the electric field intensity distribution in thecavity) in the axis direction or the stripe transverse directionascribed to interactions between longitudinal modes, gain saturation anda standing wave effect. Anyway, the coherence length of an ordinarylaser, in which no compulsory frequency modulation is carried out by anexternal cavity or the like, is far longer than the dimensions of theoptical system supposed in this invention.

In order to make the speckle pattern essentially disappear in solvingthe aforementioned problems, the sufficient condition is to provide astate in which the distribution of the scattering optical path length inthe multiple scattering region is made sufficiently longer than thecoherence length peculiar to the semiconductor laser, and the scatteredlight component outputted from the multiple scattering optical systemtimewisely loses coherence. This does not always mean that the totallength itself of the optical system is required to be longer than thecoherence length. That is, it is possible to extend the actualscattering optical path length (or its average value) a few times orseveral tens of times the geometrical length by higher-order multiplescattering or to intentionally extend the line width of thesemiconductor laser by some technique.

However, even if the timewise coherence of the semiconductor laseritself is reduced to a certain extent by adopting a special devicestructure, the reduction does not contribute to the improvement of eyesafety in the situation that it is still spatially regarded almost as apoint light source. On the other hand, it is not easy to extend thetotal optical path to or longer than the coherence length of thesemiconductor laser even when the spatial coherency is reduced byadopting a minute multiple scattering optical system as in thisinvention and repeating wave front splitting. If the parameters of thevolume of the scattering region, the type, size, dispersion density andso on of the scatterers are changed, then the speckle patternaccomplishes drastic transfiguration according to the change in thescattering optical path length distribution, and accordingly, a means tocontrol this is needed.

As described hereinabove, there exists no reference document thatclarifies the problems possibly occurring in controlling the timewiseand spatial coherency and the radiant intensity of a semiconductor laserintended for a static multiple scattering optical system in whichscatterers are dispersed at a high density in an extremely minute regioninside a three-dimensional structure and specifies a concrete indicatorand an optimization example of the parameters of the scatterer, theinfluence of the reflective surface included in the scattering opticalpath and the design of the entire multiple scattering optical system solong as the present applicant knows. This invention discloses variousmeans for extending the scattering optical path length or various meansfor more effectively reducing the spatial coherency in a minute multiplescattering optical system and achieves a speckle reduction sufficientfor securing eye safety.

In the multiple scattering optical system of the minutethree-dimensional structure, there is a variety of parameters of itsportions, and the optimization of the parameters is accompanied bydifficulties more serious than in the case of a single diffuser or thelike. For example, if the scatterer density is increased throughout theentire multiple scattering optical system in order to remove theaforementioned problems of speckles, then the skirt component trailingoutwardly of the half-value angle significantly appears in terms of theradiant intensity to the outside, and the operating power issignificantly increased, consequently losing the practicability of thelight source device. Moreover, when the scatterer density is too high,the transmitted light toward the optical axis direction is shielded, andan output usable as a light source cannot sometimes be sufficientlyobtained. This invention provides various means for not only restrainingthe speckles of the near-field pattern and the far-field pattern to alevel that poses no problem but also adjusting the angular distributionof the radiant intensity of light emitted to the outside via a multiplescattering optical system without impairing the optical output usable asa light source device.

Next, individual means for achieving the multiple scattering opticalsystem that sufficiently restrains the problems of speckles whileirreversibly expanding the near-field pattern and has the desiredoptical characteristics in a minute light source device that releasesstimulated emission of light from the semiconductor laser to the outsidevia the multiple scattering optical system that is integrated with asemiconductor laser by surrounding the laser are disclosed, and theoperation of the means will be described.

In order to accomplish the above object, the present invention providesa light source device having a light source element from which outputlight is emitted to outside via a multiple scattering optical system,wherein the multiple scattering optical system includes at least a firstregion that is located adjacent to the light source element, and asecond region that abuts on the first region and reaches the outside, ofthe first and second regions, at least the first region containsscatterers, a density of the scatterers in the first region is higherthan a density of scatterers in the second region, and the light sourcedevice has an amount of near-field pattern speckles σ_(PAR) that iswithin a range of:σ_(PAR)≧8×10⁻³.

The light source element may preferably have an optical waveguidestructure. The second region may have a lens portion. Alternatively, thesecond region may preferably serve as a magnifier for at least aprincipal portion of a secondary planar light source formed at aninterface between the first region and the second region.

According to the light source device of the above-mentionedconstruction, by generating mainly in the first region the multiplescattering that sufficiently reduces the spatial coherency of the outputlight from the light source element and controlling the angulardistribution characteristic of the radiant intensity mainly by themagnifier of the second region, the optimization of each portion can beseparately carried out. Concretely, in a typical situation in whichcoherent light is emitted with a full width at half maximum ofapproximately 5° to 20° (junction direction) and about 10° to 40° (layerdirection) into the base material of the first region located adjacentto the semiconductor laser from, for example, a high-power semiconductorlaser, the scatterers having an appropriate scattering characteristicare dispersed at an appropriate density so that in the first region thelight undergoes sufficient of multiple scattering of a frequency of notsmaller than a few times as a transport optical depth described indetail later. With this arrangement, the wave front of the laser beam isdivided into an extremely large number of parts and efficiently diffusedeven inside a minute volume, and the crossing angle between scatteringpaths statistically expands. Therefore, a local peak, which has anexpansion of about 0.01 mm to 0.1 mm that possibly causes a problemparticularly for eye safety, is scaled down and made indistinct in thenear-field pattern. The probability distribution of the amplitude of PAR(Peak-to-Average Ratio) is regarded as a Gaussian distribution, and thedeviation σ of PAR is reduced to an extremely low level of not greaterthan 10⁻¹ or not greater than 10⁻². As described above, through thesufficient multiple scattering in the first region located adjacent tothe semiconductor laser, a single secondary planar light source expandedto a finite size is formed at the interface between the first region andthe second region, and a global spatial coherency is lost when thesecondary planar light source is viewed as a whole. The laser beamradiated from the secondary planar light source is made incident on thesecond region with an almost complete Lambertian radiant intensitydistribution as a consequence of the multiple scattering.

Further, the lens portion should desirably be provided so as to form anenlarged virtual image of the object (planar light source in this case)placed in the vicinity of the front focal point as a magnifier for atleast the principal portion of the secondary planar light source. Thatis, when the secondary planar light source is observed from outside, thelight source side focal point of the magnifier is located deeper thanthe secondary planar light source so that the second region forms anerected virtual image of the secondary planar light source. Typically,the interface between the first and second regions is arranged shiftedahead of the focal point on the optical axis so that at least theprincipal portion of the secondary planar light source settles insidethe solid angle covering a span from the focal point to the lens portionof the second region.

As described above, by constituting the second region in contact withthe first region, the probability of the occurrence of local overlap ofoptical paths at a low angle can be reduced through the process ofconverting the near-field pattern of the secondary planar light sourceinto the far-field pattern in the second region while expanding thenear-field pattern by the multiple scattering and efficiently collectingalso the scattered components to a wide angle. Therefore, a beam patternthat has a high uniformity of radiant intensity and satisfactorysharpness is formed while restraining the speckles of the near-field andfar-field patterns even when a semiconductor laser is employed as thelight source element, and a preferable optical characteristic and thesecuring of eye safety can be made compatible even in a minute multiplescattering optical system.

As described above, the output light from the light source element hasits spatial coherency reduced through the first region and its radiantintensity distribution finally shaped in the lens portion owned by thesecond region and then emitted to the outside. A free space is normallysupposed as the outside of this second region. That is, the final outputlight can be emitted into a space such as the atmosphere, a vacuumchamber or the cosmic space free from an object that interrupts theoptical path. Otherwise, if the lens portion can achieve the desiredbeam shaping, the second region may be made of a medium that has anotherrefractive index, such as various resins, plastics, water and so on.Moreover, the outside of the second region may be subjected to somemodification, which is not described in detail in the presentspecification, such as shaping of the outside itself of the secondregion or an enclosure of the second region by a frame, a container orthe like. Even when the output light leaks as a consequence of theremoval or damage of such things, the eye safety of the light sourcedevice can be secured quite similarly.

Moreover, this light source device can be suitably used for a wirelessoptical communication module that incorporates a near infraredhigh-power semiconductor laser, a small-sized video projector thatincorporates a blue-violet to ultraviolet semiconductor laser formedinto a white light source by means of scatterers that have a wavelengthconversion function and so on.

In this light source device, the scatterer density allowed to beincluded in the second region of the multiple scattering optical systemshould preferably be not greater than 1/10 of the density of thescatterers included in the first region. Moreover, a filler materialdifferent from the scatterers of the first region may be dispersed at adensity deviating from the above-mentioned density range so long as thematerial does not have the function of scattering the output light fromthe light source element. Anyway, it is preferable that the output lightfrom the light source element undergoes scattering at most only a fewtimes in average or does not undergo scattering at all in the secondregion. With this arrangement, a beam optical system, in which theoperation of the multiple scattering region is effected mainly in thefirst region and of which the uniformity is satisfactory with theunnecessary skirt restrained from trailing outwardly of the half-valueangle of the radiant intensity, is constituted compatibly with thesecuring of eye safety.

In one embodiment, assuming that a size parameter q, which represents arelation between a particle size mode Ds of the scatterers and a centerwavelength λ in a base material of the first region of the light sourceelement, is expressed by:q=(2π/λ)·(Ds/2),then the particle size mode Ds of the scatterers is within a range thatallows the size parameter q to fall within a range of approximately1-50, and at least the first region includes a portion where thescatterers are dispersed at a high density so that an average nearestneighbor distance of the scatterers becomes equal to or smaller thantwenty times the particle size mode Ds of the scatterers.

It is known also from a classic scattering theory that a scatteringcharacteristic of a comparatively high isotropy and a largecross-sectional area can be obtained in a boundary between Rayleighscattering and Mie scattering. However, through detailed examinations ofthe speckle pattern in the multiple scattering optical system, it wasfound to be preferable for the scatterers of which the asymmetry of thescattering amplitude was small (backscattering components was fewalthough the isotropy was high) and in which scattering preferable forconstituting the first region was made possible to typically have a sizeparameter q of 1 to 5 and particularly mainly about 1 to 10. Otherwise,in the construction of a certain kind of minuter multiple scatteringoptical system, it is sometimes preferable that the size parameter q ismainly about 10 to 50 for an intentional increase in the forwardscattering components. Moreover, it is sometimes preferable to dispersescatterers that have different materials or particle size distributionsin mixture so that a plurality of particle size distribution peaks areprovided within the above-mentioned range. It was found to besufficiently effective to set the size parameter q like this for a lightsource element that had a continuous wavelength spectrum expansion ofseveral tens of nanometers like SLD, a semiconductor laser thatoscillated in multiple modes over several tens of nanometers or thelike. That is, by setting the size parameter q to the center wavelengthof the output light, the spatial coherency can be reduced extremelyeffectively, and the near-field pattern can be irreversibly expanded.

With regard to the absolute value Δn of the refractive index differencebetween the scatterers and the medium in the first region, mainly usedscatterers should provide typically a value of not smaller than 0.1 ormore and desirably a value of not smaller than 0.15. In some cases,preferable result can be obtained by mainly using scatterers with whichthe refractive index difference Δn is not smaller than about 0.05 in acertain kind of multiple scattering optical system. Furthermore, it ispreferable to select the size parameter q such that the product Δn·qfalls within a range of approximately 2 to 8 and especially assumes avalue near 3. Since the angular distribution of the scattering amplitudeof each individual scatterer does not strongly depend on the refractiveindex difference Δn, there is given a criterion to select the desirableparticle species that have a small asymmetry of the scattering amplitudeand also a comparatively small amount of backscattering components.

The parameters of the scatterers will be described together with theembodiments. The above-mentioned numerical limitations are caused mainlyfrom difficulties in the minuteness of the multiple scattering opticalsystem or a high-density uniform dispersion there and include anessential problem that should be solved inclusive of the construction ofthe entire multiple scattering optical system.

Furthermore, it is desired to set the scattering mean free path to about10 μm or less in order to obtain the aforementioned sufficient multiplescattering inside the first region, which typically has an optical axisdirection dimension L of one millimeter to several millimeters, in themultiple scattering optical system. As a general rule, the scatteringcross-sectional area as of a single scatterer is obtained from thescattering theory, and the scattering mean free path l can be estimatedfrom:l=1/(σs·Ns)(where Ns represents the number of scatterers per unit volume, or thenumber density), i.e., from the dispersion density (volume ratio orweight ratio).

Accordingly, the ratio L/1 is made a parameter as an average or meanscattering frequency to be an index with a satisfactory attenuation of aballistic straight light component in the optimization process of themultiple scattering region. On the other hand, according to thescattering angle θ and the phase function p(θ) of the single body of thescatterer, an asymmetry factor g (mean cosine) is obtained from:

g = ⟨cos  θ⟩ = ∫₀^(π)cos  θ ⋅ p(θ) ⋅ 2π sin  θ 𝕕θ, where  ∫₀^(π)p(θ) ⋅ 2π sin  θ 𝕕θ = 1and the transport average or mean free path l_(AVE) can be defined by:l _(AVE) =l/(1−g)By using this relation and making the transport optical depth L/l_(AVE)a parameter, there can be provided an index of collapsing process ofcoherency due to the propagation of light in the optical axis directioninside the multiple scattering region. Particularly, in this lightsource device, it was discovered that the combinational conditions ofthe portions that reduced the speckles to an extremely low level andsatisfied the Class 1 eye safety was able to be extracted by employingthe scatterers satisfying the desirable size parameter q and using thetransport optical depth L/l_(AVE) as an index.

In this case, the multiple scattering optical system of this lightsource device can be desirably constituted by the aforementioned methodon the basis of the fact that the scattering mean free path l is reducedas the scatterer density or the volume ratio is increased so long as thescatterers are spatially distributed at random. Typically, a preferablemanufacturing condition can be found within a dispersion density rangeof about 0.5 vol % to 30 vol %. However, in the case of high densityscatterers in which the scatterers can be brought in direct contact withone another, an undesirable phenomenon of possibly occurs from theviewpoint of speckle reduction. One cause is a problem of the secondarycohesion of the particles, and the other cause is the fact that therandomness is reduced due to the dense structure of the spatialdistribution of the scatterers. It is important for managing the actualmanufacturing process to clarify the necessary minimum dispersiondensity within the desired range of the scatterer particle size in orderto efficiently reduce the spatial coherency by means of an opticalsystem that is as minute as possible.

Accordingly, it was found that, by distributing the average nearestneighbor distance of the scatterers mainly dispersed in the first regionso that the distance falls within about twenty times the particle sizemode Ds of the scatterers, eye safety was able to be secured bygenerating multiple scattering a few times to several hundreds of timesas the aforementioned transport optical depth L/l_(AVE) in amillimeter-order extremely minute multiple scattering optical system.

In one embodiment, the first region employs a gel-like or rubber-likematerial as the base material.

According to the light source device of the above-mentioned embodiment,by using a gel-like material or a rubber-like material (elastomer or thelike) of which the hardness after hardening is specified by theso-called penetration and JIS A hardness, the material is hardened asthe first region. With this arrangement, the change with a lapse of timeof the scattering characteristic due to the subsidence of the scatterersor the like is prevented, and the first region can be stably retainedeven when the second region is formed through the press-fitting processof resin sealing or the like as in the transfer molding process.

Moreover, by using a gel-like or rubber-like material as the basematerial, easily available various scatterers can be preferablyuniformly dispersed also by means of a simple kneading and dispersingdevice that does not have a strong shear force. That is, by dispersingthe scatterers having the size parameter q within a range of 1 to 50within a density range of not greater than about 0.5 vol % to 30 vol %in the gel-like or rubber-like material, there can be found amanufacturing condition capable of obtaining a satisfactory dispersioncondition in which the average nearest neighbor distance is made withinabout twenty times the particle size mode Ds. Typically, a volume ratioof 1 vol % to 15 vol % of the scatterers with respect to the basematerial is extremely effective for speckle reduction. Moreover,particularly the polymer particles, such as acrylic, styrenic, andmodified silicone particles, sometimes have an extremely preferablecharacteristic as the scatterer of this invention, and it is possible tosecure an appropriate refractive index difference and a satisfactorydispersibility to the general silicone-based gel and elastomer.

Particularly, the silicone gel, which has a comparatively highflowability before hardening and a sufficient flexibility after thehardening, is more preferable as the scattering base material. Asilicone gel, which has a viscosity of not higher than about 6000 mPa·sbefore hardening, can be used extremely suitably for the inexpensivedispersion and kneading device of, for example, a vessel-rotating typemixer or the like. Moreover, it is preferable to increase the deviationof the particle size distribution of the scatterers dispersed at a highdensity within the desirable range of the size parameter in order tokeep appropriate the hardness (softness) after the hardening of basematerial and to obtain a uniform monodispersion.

As described above, by arranging the gel-like or rubber-like material,in which the scatterers are properly dispersed at a high density and anappropriate flowability is possessed, adjacent to the light sourceelement and hardening the material with its volume and three-dimensionalconfiguration shaped into the desired states, there can be obtained anextremely preferable characteristic of the first region of the multiplescattering optical system. That is, it becomes possible to efficientlylose the spatial coherency of the output light in the minute volume ofthe neighborhood of the light source element.

In addition to the aforementioned effects, the following various effectscan be obtained. That is, a stress depending on a difference in thethermal expansion coefficient that the light source element receivesfrom other portions of the optical system is alleviated by employing thegel-like or rubber-like material as the base material, and thereliability during the high-power operation can be secured with animproved heat radiation property. Above all, when a semiconductor laseris employed, it becomes possible to generate a coherent backscatteringpeak occurring on the optical axis, i.e., return light to the laseremitting end surface of the semiconductor laser and control the quantityof light to a certain extent by means of scatterers (particularly bydispersion density). Typically, it was found that the timewise coherencyof the semiconductor laser itself was able to be reduced by moderatelypromoting the increase in the spectral linewidth or the longitudinalmultimode of the semiconductor laser within a range of a scatteringvolume ratio (dispersion density) of about 1 vol % to 30 vol %. However,there is a tendency that it is difficult to produce a high power due totypically the instability of laser oscillation caused by the intensehiding power of the scattering region when the dispersion density of thescatterers exceeds 30 vol %.

In one embodiment, the light source device includes a recess portionhaving a wall surface and a bottom surface that define the first region,wherein a metal layer is formed on at least part of the wall surfaceand/or of the bottom surface, and the light source element is directlyor indirectly fixed to the bottom surface, and a surface of the metallayer formed on the at least part of the wall surface and/or of thebottom surface of the recess portion serves as a reflective surface toscattered light of the output light from the light source element.

According to the light source device of the above-mentioned embodiment,by providing the recess portion that has the wall surface and the bottomsurface defining the configuration of the first region, the dimensionsof the volume, the three-dimensional configuration and so on of thefirst region can be distinctly defined even when the first region forsufficiently reducing the timewise or spatial coherency is constructedof the gel-like or rubber-like fluid material. That is, a secondaryplanar light source, which has a high uniformity of intensitydistribution and of which the size is definite, can be obtained bycontrolling the scatterer density of the first region.

Moreover, particularly the metal layer constitutes at least part of thewall surface and/or of the bottom surface of the recess portion, wherebythe multiply scattered light generated in the first region that isformed and retained inside it is guided as a wave toward the secondregion through a dispersion port provided by the opening of the recessportion although the light is confined in the first region as a whole.Through this process, the effect that the spatial coherency isparticularly remarkably reduced is produced.

In order to obtain the effect of making definite the light source sizeby improving the intensity distribution uniformity of this light sourcedevice and the effect of efficiently reducing the spatial coherencywithout impairing the output light, the reflectance of light incident onthe metal layer of the recess portion is desired to be high with respectto every incidence angle. In general, the smaller the refractive index(complex refractive index real part), the smaller the critical angle ofthe total reflection is. Also, the greater the extinction coefficient(or the absolute value of the complex refractive index imaginary part),the higher the reflectance is obtained even within the critical angle.Therefore, the typically preferable materials that constitute the metallayer of the recess portion are the metals of gold, silver, copper andso on.

Particularly, when the silicone gel is served as the base material ofthe first region and the metal layer is made of silver, there can beobtained a total reflection critical angle of smaller than 10° and alarge reflectance of not smaller than 90% within the critical angle withrespect to the visible and infrared light. Therefore, it is practicallyextremely preferable to form the metal layer of the recess portion by anAg-based plating process or an Ag paste process, and it is alsopreferable to carry out a plating process containing magnesium for thesame reasons. In this case, there is no problem of the opticalcharacteristic even if the surface of the metal layer is covered with avery thin natural oxide or the like, and it is needless to say thatsatisfactory electrical continuity can be obtained by the normal processof die bonding, wire bonding or the like.

In one embodiment, the metal layer on the at least part of the wallsurface and/or of the bottom surface that define the first region iscontinuously formed so that substances other than the metal are notexposed in a principal portion positioned within reach of the scatteredlight spatially distributed in the first region.

That is, not only when the metal layer is constructed of a single layerbut also when the metal layer is constructed of a plurality of layers toform a composite recess portion, the single metal layer or the pluralityof metal layers are continuously formed so that other material notexhibiting the total reflection characteristic is not brought in directcontact with the principal portion. By thus constituting the firstregion, the scattered light spatially distributed in the first regioncan be effectively prevented from leaking in directions other than theoptical axis direction. Therefore, the function of improving theintensity distribution uniformity of the light source device and makingdefinite the light source size possessed by the first region and thefunction of efficiently reducing the spatial coherency can be obtainedwithout causing the loss of the optical output.

In one embodiment, the surface of the metal layer formed on at leastpart of the wall surface of the recess portion serves as a reflectivesurface that changes an optical axis direction of an outgoing beam ofthe light source element toward an interface between the first andsecond regions, and the size parameter q of the first region fallswithin a range of approximately 1 to 15.

According to the light source device of the above-mentioned embodiment,the scattering optical path length can be extended by bending theoptical axis of the first region one or more times as a whole by thesurface of the metal layer formed on at least part of the wall surfaceof the recess portion for the generation of an increased amount ofscattering. Moreover, at least initial scattering (irregular reflection)is caused by the reflection surface obtained through a simple processwithout considering the mirror surface of wavelength accuracy.Therefore, a multiple scattering operation sufficiently reducing thespeckles can be obtained by a multiple scattering optical system ofsmaller dimensions (size), which meets the earnest demand for reductionin size.

In this case, the probability of the existence of a light rayapproximately parallel to the optical axis of the magnifier of thesecond region is relatively increased immediately after the conversionof the optical axis in the first region. Therefore, the scatterersmainly dispersed in the first region should desirably have acomparatively small particle size of a size parameter q within a rangeof 1 to 15 and particularly within a range of 1 to 10, so that theasymmetry factor g (mean cosine) is apart from one (1) and becomes closeto or in excess of the complete Lambertian value (2/3). With thisarrangement, an optical output capable of being utilized as a lightsource can be efficiently obtained by effectively attenuating theballistic straight light component and restraining the backscatteringcomponent. The scatterers should preferably have a refractive indexdifference Δn of not smaller than 0.1 and particularly not smaller than0.15 with respect to the base material of the first region in makingcompatible the requirement for the angle dependence of the scatteringamplitude with the standardization scattering cross-sectional area. Asdescribed in detail later, the value of g and the difference of Δn canbe absorbed to a certain extent by the setting of the dispersiondensity, and the desired characteristics can be obtained by setting thevalues within, for example, the aforementioned range. In particular, therange of the size parameter q is important.

The construction mainly including the scatterers of a comparativelysmall particle size is able to most easily secure eye safety and obtainsatisfactory light source characteristics free from the influence of theoptical loss due to the hiding power or the like of the scatterers solong as a combination of the base material capable of high densitydispersion with a kneading device can be utilized. Moreover, a generalend surface emitting type semiconductor laser capable of achieving highpower operation can be mounted in the form of a simple die bonding, andtherefore, a light source device that has the aforementionedcharacteristics can be manufactured at extremely low cost.

As a modification example the first region, it is acceptable to mix anddisperse scatterers of a relatively large particle size of a sizeparameter q deviating from the range of approximately 1 to 15 with theaforementioned scatterers serving as main scatters (maximum in terms ofnumber density). The scatterers are dispersed so that the averagenearest neighbor distance of all the scatterers falls withinapproximately twenty times the mode diameter of all the scatterers, orthe mode diameter Ds of the scatterers mainly dispersed. With thisarrangement, the dispersion condition, which improves the outputefficiency while satisfying eye safety and simplifies the dispersionprocess, is found. Moreover, it is also desirable to constitute thefirst region as a laminate constructed of two or more layers byspatially separating the scatterers that have different parameters orspatially changing the dispersion density of the same scatterers in thefirst region of which the configuration is determined by the recessportion, instead of carrying out the mixed dispersion. A sufficientspeckle reducing effect can be obtained while restraining the hidingpower by constituting the first region of a plurality of portions (orlayers) and changing the volumes of the portion mainly intended for theoperation of dividing the coherent wave front into a plurality ofportions and the portion mainly intended to generate scattering of ahigh symmetry a plurality of times. Forming the first region into amultilayer would be disadvantageous from the viewpoint of the total costalthough the difficulties in optimizing the dispersion process arereduced.

In one embodiment, the surface of the metal layer formed on at leastpart of the wall surface of the recess portion serves as a reflectivesurface that changes an optical axis direction of an outgoing beam ofthe light source element a plurality of times, and the size parameter qof the first region falls within a range of approximately 10 to 50.

According to the light source device of the above-mentioned embodiment,by providing a construction in which the outgoing beam (i.e., directwave) of the light source element reaches the opening of the recessportion after two to five times of reflection when it is assumed that aspecific inclination angle is given to the principal portion of the wallsurface of the recess portion and no scatterer exists, the scatteringoptical path length can be set large. This arrangement is able tomaintain the output efficiency high while effectively generatingmultiple scattering in a comparatively small volume particularly whenthe thickness in the optical axis direction of the second region is thinand to achieve speckle reduction by remarkably improving the uniformityof the secondary planar light source formed at the interface with thesecond region.

Moreover, in the above-mentioned construction, the size parameter q ofthe scatterers mainly dispersed in the first region is within a range ofapproximately 10 to 50 and particularly within a range of 15 to 40. Thatis, the scatterers should preferably have a comparatively large particlesize and an asymmetry factor g exceeding 0.9. With this arrangement, thenear-field pattern can be more effectively uniformed by the multipathreflection on the wall surface and the multiple scattering of thescatterers that have a comparatively intense forward scatteringcharacteristic, and the size of the apparent light source can easily beexpanded. A scatterer of a comparatively low refractive index differenceΔn may be employed, and acrylic organic particles, SiO₂, other metaloxides and so on can be suitably used in association with the basematerial of the silicone base. For example, the size parameter q of 10to 50 with respect to the near infrared semiconductor laser thatoscillates with a wavelength of 900 nm in the air corresponds to aparticle size Ds of about 1 μm to 7 μm. When the scatterers of such acomparatively large particle size are mainly dispersed, a minutemultiple scattering optical system can easily be constituted by asimpler kneading and dispersion process. Moreover, the aforementionedconstruction is able to mount an end surface emitting type semiconductorlaser capable of achieving high power operation in the form of a simpledie bonding and to manufacture the light source device that has thedesired characteristics at low cost.

Of course, in the above-mentioned construction, TiO₂ of a largerefractive index difference or the like can be suitably used as the mainscatterers so long as the scatterers are particles of a comparativelylarge particle size. However, inorganic oxide based particles of a largeparticle size having a high refractive index difference tends to becomeextremely expensive. In a construction in which the combination ofmultipath reflection and multiple scattering is used, it is extremelypreferable to mix and disperse the main scatterers of a comparativelylow refractive index difference (typically about 0.05≦Δn≦0.2) and alarge particle size (generally q≧10) together with the subordinatescatterers of a high refractive index difference (generally Δn≧0.2) anda small diameter (generally q≦10) like the aforementioned TiO₂. Bysetting the average nearest neighbor distance including all thescatterers within the range of approximately twenty times the particlesize mode, or the mode Ds of the scatterers mainly dispersed andblending the scatterers at a comparatively low density within thepermissible range from the viewpoint of speckle reduction, theuniformity of the secondary planar light source and the outputefficiency can be made compatible with speckle reduction.

It is acceptable to constitute the first region as a laminateconstructed of two or more layers by spatially separating the scatterersthat have different parameters or changing the dispersion density of thesame scatterers in the first region instead of carrying out the mixeddispersion. For example, it is acceptable to constitute the greater partof the region located adjacent to the light source element (e.g.,semiconductor laser) in the first region of a layer in which scatterersof a comparatively large particle size are dispersed at a density of 10vol % and provide a layer in which scatterers of a comparatively smalldiameter and a high refractive index difference are dispersed at adensity of 1 vol % in the upper portion of the layer. Even if theuppermost layer of the first region is a polydispersion including anagglomerate, there can be found a dispersion condition that thepreferable speckle reducing effect, light output efficiency and theflattening of the near-field pattern are made compatible. Theconstruction of the first region can be thus optimized by constitutingthe first region of a plurality of layers or portions and changing thevolumes (or volume ratio) of the portion mainly intended for theoperation of dividing the coherent wave front into a plurality ofportions and the portion mainly intended to generate highly balancedscattering a plurality of times, whereas it becomes disadvantageous fromthe viewpoint of the total cost.

In one embodiment, an opening of the recess portion has a diameterlarger than that of the bottom surface, and

assuming that a ratio of a depth to the diameter of the bottom surfaceof the recess portion is given as an aspect ratio, r, and an angle madebetween a normal line of the wall surface of the recess portion and theoptical axis of the outgoing beam of the light source element is θ[deg], then a condition expressed by:max{2r, 3}≦θ≦20ris satisfied.

According to the light source device of the above-mentioned embodiment,the diameter of the opening of the recess portion is larger than that ofthe bottom surface. By satisfying the above-mentioned condition, specklereduction can be achieved by maintaining a high output efficiency whileeffectively generating multiple scattering and remarkably improving theuniformity of the secondary planar light source formed at the interfacewith the second region.

Moreover, the angle θ should preferably be provided with a lower limit.That is, when there is multiple scattering of which the optical depthexceeds several hundreds of times, the optical output tends to becomehard to take out as a consequence of its being hidden. The phenomenon oflight being not reflected on but absorbed into the metal layerconstituting the wall surface statistically becomes unignorable when thefrequency of incidence on the wall surface of the recess portion becomesextremely large, and efficiently guiding light to the second regiontends to become difficult. To solve it, it was found that the problem ofan excessive increase in the average frequency of the multiplescattering is practically avoidable by setting 2r≦θ. According to thecombination of a typical metal that constitutes the outermost surface ofthe wall surface of the recess portion with a silicone-based gel orother resin-based material typical as the base material of the firstregion, the critical angle of total reflection becomes about 5° to 20°.In a typical construction example of the multiple scattering opticalsystem, it was found that the problem of the excessive increase in theaverage frequency of incidence on the metal layer within the criticalangle was practically avoidable by setting 3≦θ [deg]. Therefore, bysetting max{2r,3}≦θ, i.e., by setting the angle θ equal to or greaterthan (2r)° or 3° whichever is larger, the problem of difficulty inefficiently guiding light to the second region is solvable.

By making the wall surface of the recess portion have a completely idealgeometric configuration, the angle θ might simply be obtained by ageometric calculation. However, in this light source device, it isimportant to constitute all the elements of the multiple scatteringoptical system by simpler means. For example, when the constituentelements of the recess portion are formed by the process of drilling aresin substrate by a body of rotation, indentation or punching of acolumnar or polygonal pillar or the like, deviation from the idealconfiguration and dimensions often occurs particularly in theneighborhoods of the bottom surface and the opening. The aforementionednumeric limitations of θ≦20r and further max{2r,3}≦θ were obtained byevaluating the non-defective product yield in consideration of theactual manufacturing processes as described above.

In one embodiment, at least part of the wall surface of the recessportion forms a cylinder whose top and bottom have approximately samesectional configurations, and assuming that a ratio of a depth to adiameter of the cylinder of the recess portion is given as an aspectratio, r, and an angle made between a normal line of the wall surface ofthe recess portion and the optical axis of the outgoing beam of thelight source element is θ [deg], then a condition expressed by:max{a tan(r/5), 3}≦θ≦a tan(r/2)is satisfied.

According to the light source device of the above-mentioned embodiment,in at least one principal portion of the wall surface constituting therecess portion in the first region as a structure for converting theoptical axis direction of the outgoing beam of the light source elementa plurality of times, the cross-sectional configurations of the openingportion and the bottom surface portion are almost the same cylindricalconfigurations. Assuming that the aspect ratio (depth/diameter in theoptical axis direction of the light source element) of the recessportion is r and an angle made between the normal line of the wallsurface and the original optical axis of the light source element is θ[deg], it is acceptable to satisfy the relation:a tan(r/5)≦θ≦a tan(r/2)and arrange at least the principal portion of the wall surface thatconstitutes the recess portion relatively inclined with respect to theoptical axis of the outgoing beam of the light source element. Althoughthe angle θ can take an extremely small value depending on the value ofthe aspect ratio, it is desirable to provide the angle θ of not smallerthan 3° in order to avoid the problem of the excessive increase in theaverage frequency of incidence on the metal layer within the criticalangle as already described. Moreover, it is needless to say that therecess portion is not always required to have a circular cross-sectionalconfiguration, the configuration is desired to be axisymmetric to theoptical axis of the light source element, and that the mutually opposedsides should be parallel to each other in a cross section that includesthe axis of the wall surface configuration of the recess portion and theoptical axis of the light source element.

In the above-mentioned construction, the size parameter q of thescatterers mainly dispersed in the first region may be within the rangeof approximately 10 to 50 and particularly within the range of 15 to 40.That is, the scatterer should preferably have a comparatively largeparticle size and an asymmetry factor g close to one. With thisarrangement, the near-field pattern is more effectively uniformed by themultipath reflection on the wall surface and the multiple scattering ofthe scatterers that have a comparatively intense forward scatteringcharacteristic, and the size of the apparent light source can easily beexpanded. Moreover, a scatterer of a comparatively low refractive indexdifference Δn can also be employed.

As described hereinabove, it is preferable to mix and disperse the mainscatterers of a comparatively low refractive index difference (typically0.05≦Δn≦0.2) and a large particle size (q≧approx. 10) together with thesubordinate scatterers of a comparatively high refractive indexdifference (Δn≧approx. 0.2) and a small diameter (q≦approx. 10) withinthe permissible range from the viewpoint of the speckle reduction inallowing the speckle reduction with an improved dispersion uniformitywhile improving the uniforming of the secondary planar light source andthe output efficiency. Otherwise, it is also acceptable to constitutethe first region of a laminate constructed of two or a plurality oflayers by arranging the scatterers that have different parametersspatially separately or changing the dispersion density instead ofcarrying out the mixed dispersion.

In one embodiment, the light source element is a semiconductor laser.

By applying the eye-safe means of an extremely small optical loss asdescribed above, the electric power efficiency of various conventionallight source devices that employ an LED for the light source element canbe improved by at least two to three times or more. Furthermore,according to the eye-safe means in the minute region, the originalresponse of the laser is not impaired in at least a frequency region ofnot higher than about several gigahertz. Therefore, a small-sized,light-weight low-cost transceiver for wireless optical communications,which has not existed, can easily be provided.

In one embodiment, the semiconductor laser has an active layer includingan InGaAs layer on a GaAs substrate and an emission wavelength within arange of from 880 nm to 920 nm inclusive.

According to the light source device of the above-mentioned embodiment,the light in the wavelength band of 880 nm to 920 nm induced and emittedfrom the semiconductor laser that has the active layer including theInGaAs layer on the GaAs substrate has a wavelength close to the peaksensitivity wavelength of a Si photodiode which is a typicalphotodetector, and this light source device is suitable as atransmission means for optical communications. Moreover, the thresholdcurrent and the temperature characteristic of the semiconductor laserare remarkably improved in comparison with those of the semiconductorlight-emitting device of the 780-nm band or the like. Therefore, thislight source device allows achievement of an optical communicationmodule which satisfies the Class 1 eye safety for wireless opticalcommunications and concurrently is inexpensive and excellent inelectrical and optical characteristics.

Moreover, particularly, by constituting the layers that have a highoptical density, such as, for example, a quantum barrier layer and alight guide layer located adjacent to the InGaAs layer or a light guidelayer provided besides the active layer, of at least one of a ternarylayer or a quaternary layer expressed by In_(X)Ga_(1-X)As_(Y)P_(1-Y)(0≦X<1, 0<Y<1) to thereby make those layers Al free, it becomes possibleto provide an eye-safe light source device of which the output can bemade highest in the wavelength band of 880 nm to 920 nm. Therefore, byemploying this light source device, there can be constituted an opticalcommunication module that satisfies the Class 1 eye safety andconcurrently is irrespective and excellent in electrical and opticalcharacteristics for wireless optical communications.

In one embodiment, the semiconductor laser has spatial fluctuations inat least one of its composition or its layer thickness.

According to the light source device of the above-mentioned embodiment,by forming a pseudo gain grating by intentionally carrying outisland-shaped three-dimensional growth particularly for remarkablygenerating its thickness distribution during the crystal growth of theactive layer and other layers of the semiconductor laser, the spectrallinewidth during the laser operation can be extended. Moreover, byforming a pseudo refractive-index grating by carrying out growth forintentionally causing local compositional fluctuations during thecrystal growth of the quantum well barrier layer, the light guide layerand so on, the spectral linewidth can be extended. Therefore, thesemiconductor laser, which has the fluctuations in the composition orthe layer thickness, is effective for speckle reduction.

Moreover, in one embodiment, the semiconductor laser has the activelayer including the InGaAs layer on the GaAs substrate and includes aternary layer or a quaternary layer expressed byIn_(X)Ga_(1-X)As_(Y)P_(1-Y) (0≦X<1, 0<Y<1), and at least one of thelayers has fluctuations in at least one of the composition and the layerthickness.

According to the light source device of the above-mentioned embodiment,the following remarkable effects can be obtained by employing the activelayer including the InGaAs layer, further employing a GaAsP ternarymaterial or the InGaAsP quaternary material as a quantum well barrierlayer or employing an InGaAsP quaternary material that has latticematching with GaAs, as a light guide layer.

That is, by forming a pseudo gain-coupled grating by intentionallycarrying out island-shaped three-dimensional growth particularly forremarkably generating its thickness distribution during the crystalgrowth of the InGaAs layer formed as a quantum well, the spectrallinewidth during the laser operation can be extended. Moreover, byforming a pseudo index-coupled grating by carrying out growth forintentionally causing local compositional fluctuations during thecrystal growth of the GaAsP layer or the InGaAsP layer formed as aquantum well barrier layer or a light guide layer, the spectrallinewidth can be extended. It was confirmed from the results ofobserving the visibility (criteria representing a contrast ofinterference fringes and so on) through interference experiments thatthe timewise coherency of the output light emitted from thesemiconductor laser having the pseudo grating of an obscure phase in itscavity was able to be reduced by one or more orders of magnitude incomparison with that of ordinary high-power semiconductor lasers.Therefore, the semiconductor laser, which has such fluctuations in thecomposition or layer thickness, is preferably concurrently used as otherconstituent elements of this high-power light source device togetherwith the first region and the second region of the multiple scatteringoptical system.

In one embodiment, at least part of a wire directly or indirectlyconnected to the semiconductor laser exists in the second region.

According to the light source device of the above-mentioned embodiment,by virtue of the arrangement that at least part of the wire directly orindirectly connected to the semiconductor laser exists in the secondregion, the wire is peeled off together with the second region anddisconnected if the second region is damaged or peeled off.Consequently, the electrification to the semiconductor laser isinterrupted, so that the laser beam of the coherency maintained at ahigh level can be prevented from directly entering the user's eyes.Although the above-mentioned operation is effective also when damageoccurs during the operation of the light source device, it is needlessto say that similar operation is effected on an attempt to use the lightsource device after the damage has once occurred.

In one embodiment, assuming that a transport mean free path of thescatterers is l_(AVE) and a dimension in the optical axis direction ofthe first region is L, then a transport optical depth L/l_(AVE) isapproximately 1 to 100.

In one embodiment, the amount of near-field pattern speckles σ_(PAR) iswithin a range expressed by:σ_(PAR)≦3×10⁻¹.

In one embodiment, the light source element has an optical waveguidestructure.

Moreover, an optical communication module of this invention ischaracterized in that the aforementioned light source device is used asa the transmission means.

According to the optical communication module, by employing the lightsource device as the transmission means and further employing, forexample, an Si photodiode as a light reception means, there can beprovided an optical communication module that satisfies the Class 1 eyesafety and concurrently is most inexpensive and excellent in electricoptical characteristics for wireless optical communications. Moreover,particularly in an optical communication module, the first region of themultiple scattering optical system is formed as a minute region locatedadjacent to the light source element (semiconductor laser). Therefore,even when the device is integrated with or formed into an integratedmodule with a photodiode, the reception system does not suffer thedisadvantages of sensitivity degradation and so on. Therefore, byforming an optical communication module by a combination of aninexpensive Si photodiode with the light source device of thisinvention, there can be provided an optical communication module thatconcurrently achieves a small size and low cost equivalent to those ofthe existing IrDA transceiver and a high-speed property and a widecommunication area equivalent or superior to those of the existingoptical wireless LAN product and is optimum for wireless opticalcommunications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining the outline of the multiple scatteringoptical system of an eye-safe light source device as the light sourcedevice of this invention;

FIG. 2 is a sectional view showing the construction of the eye-safelight source device of a first embodiment of this invention;

FIGS. 3A, 3B, 3C, 3D, 3E, 3F and 3G are graphs comprehensively showingdata concerning the near-field pattern of the above eye-safe lightsource device;

FIG. 4 is a sectional view showing the construction of the eye-safelight source device of a second embodiment of this invention;

FIGS. 5A, 5B, 5C and 5D are graphs comprehensively showing dataconcerning the near-field pattern of the above eye-safe light sourcedevice;

FIG. 6A is a sectional view showing a construction of the eye-safe lightsource device of a third embodiment of this invention;

FIG. 6B is a schematic view showing the optical path of a semiconductorlaser;

FIG. 6C is a graph showing the relative light intensity distribution ofthe near-field pattern;

FIG. 7A is a sectional view showing another construction of the eye-safelight source device of the third embodiment;

FIG. 7B is a graph showing the relative light intensity distribution ofthe near-field pattern of the eye-safe light source device shown in FIG.7A;

FIG. 7C is a sectional view showing another construction of the eye-safelight source device of the third embodiment;

FIG. 7D is a graph showing the relative light intensity distribution ofthe near-field pattern of the eye-safe light source device shown in FIG.7C;

FIG. 8 is a graph showing the results of comprehensively evaluating thenear-field pattern in the eye-safe light source devices shown in FIGS.6A, 7A and 7C;

FIGS. 9A, 9B and 9C are graphs showing the results of evaluating thefull width at half maximum and the amount of speckles of the far-fieldpattern in the eye-safe light source devices shown in FIGS. 6A, 7A and7C;

FIG. 10A is a sectional view showing a construction of the eye-safelight source device of a fourth embodiment of this invention;

FIG. 10B is a graph showing the relative light intensity distribution ofthe near-field pattern of the above eye-safe light source device;

FIG. 11A is a sectional view showing another construction of theeye-safe light source device of the fourth embodiment of this invention;

FIG. 11B is a graph showing the relative light intensity distribution ofthe near-field pattern of the above eye-safe light source device;

FIG. 12 is a graph comprehensively showing data concerning thenear-field pattern of the eye-safe light source device shown in FIG.11A;

FIGS. 13A and 13B are views showing the structure of a high-powersemiconductor laser particularly preferable for the above eye-safe lightsource device;

FIG. 14 is a sectional view showing the construction of the eye-safelight source device of a fifth embodiment of this invention;

FIG. 15A is a sectional view showing the construction of an opticalcommunication module in which the above eye-safe light source device isemployed;

FIG. 15B is a graph showing the current-to-optical output characteristicat the room temperature of the transmission section of the above opticalcommunication module; and

FIG. 16 is a view showing the construction of a conventional lightsource device in which a diffuser is employed.

BEST MODE FOR CARRYING OUT THE INVENTION

The outline of the basic construction of the multiple scattering opticalsystem of this invention will be described first with reference to FIG.1, and thereafter, typical examples, speckle pattern reduction examplesand the evaluation criteria and so on for securing eye safety in theembodiments of the light source device of this invention will bedescribed.

As shown in FIG. 1, an optical output from a semiconductor laser (notshown) is emitted typically from a micrometer-order spot 100 and has adirectivity in the direction of an optical axis 101. Scatterers 103 areuniformly distributed at a high density in a first region 102 locatedadjacently so as to surround the scatterers. In FIG. 1, stimulatedemission of light with timewisely and spatially high coherency from thesemiconductor laser travels entirely in the direction of an optical axis106 of a second region 104 while being scattered by undergoing multiplescattering in this first region 102. Then, a spot 105, which becomes asingle secondary planar light source, is formed at the interface withthe second region 104.

The multiple scattering in the region 102 is a random and staticprocess, and the distribution of the degree of spatial coherence at theinterface between the first region 102 and the second region 104reflects the original angular distribution of the radiant intensity ofthe semiconductor laser, the constituent elements (combination of thebase material and the scattering material) of the first region 102, thedimensions of the portions and so on. The spot 105 has globally lost thespatial coherency when viewed as the entire spot 105 so as not to causea problem from the viewpoint of eye safety. Moreover, the spot 105 isformed to have a finite expansion smaller than the diameter of the lensportion 104 a that serves as a magnifier at the interface between thefirst region 102 and the second region 104 so as to be regarded as asecondary planar light source that has an almost uniform near-fieldpattern. As the result of the multiple scattering in the first region102, luminous fluxes radiated in all directions from the elements of thesecondary planar light source have their own far fields behaving likealmost complete Lambertian.

The second region 104 on which the scattered light from the secondaryplanar light source is incident is provided with a lens portion 104 athat has an optical axis 106 in which the distance between the freespace and the interface and the interface configuration are determinedon the basis of the refractive index of the media so as to operate as amagnifier at least for the principal portion (e.g., a region that has anintensity of not smaller than 1/e² of peak intensity) of the spot 105. Afocal point on the light source side of the lens portion 104 a isarranged on the light source element (semiconductor laser) side of theinterface in order to form an erected virtual image of the spot 105 whenthe light source element (semiconductor laser) is observed from the freespace side and to reduce the probability that the output light from thespot 105 intersects at a low angle inside the second region 104.Moreover, there is almost no need to consider the distortion of thevirtual image in the applications supposed by this invention.

As described above, the light source device of this invention is a lightsource device in which the stimulated emission from, for example, asemiconductor laser is emitted into the free space via a multiplescattering optical system and is constituted so that the multiplescattering optical system is constructed of the first region 102 that isadjacent to the semiconductor laser and the second region 104 that isadjacent to the first region 102 and reaches a free space, the firstregion 102 contains scatterers at a density higher than that of thesecond region 104, and the second region 104 has a magnifier for atleast a principal portion of the secondary planar light source formed atthe interface between the first and second regions 102 and 104.Typically, it is assumed that the thickness in the optical axisdirection of the first region 102 is about 1 mm to 3 mm and thethickness in the optical axis direction of the second region 104 iswithin a range of about 2 mm to 10 mm depending on the configuration ofthe lens portion 104 a. FIG. 1 is a conceptual diagram for explainingthe outline of the objective optical system of this invention, andvarious concrete numerical values described later are applied to thedetailed parameters of portions, such as the scatterers, the overalldimensions and so on.

Although the eye-safe light source device in which the first region 102has a tabular construction is shown in FIG. 1, there is provided noconstituent element for limiting the configuration of the first region102 to about the expansion of the spot 105 in the intraplanar direction.For example, a tabular first region can easily be formed by providing aframe that has an expansion larger than the supposed spot size in thesurroundings of the semiconductor laser mounted on a resin substrate anddripping liquid, gel or rubber material in which scatterers aredispersed into this frame or in a similar manner. However, according tothe construction, an element or the like having another function cannotbe adjacently arranged, and a size reduction in the planar directionbecomes difficult. A reduction in the light concentrating efficiencypossibly occurs if the first region is formed additionally on aphotodetector such as a photodiode to be combined as a module capable oftransmission and reception, and accordingly, there arises a necessityfor avoiding the arrangement adjacent to the light source section.

An eye-safe light source device that serves as the light source deviceof this invention and an optical communication module that employs thedevice will be described in detail below on the basis of the embodimentsshown in the drawings.

FIRST EMBODIMENT

Dissimilarly from FIG. 1, the first region is also allowed to have asufficient thickness and intraplanar expansion by directly dripping anappropriate amount of substance that has a comparatively high viscosityin which scatterers are distributed only at the periphery of thesemiconductor laser chip.

FIG. 2 is a sectional view showing the construction of an eye-safe lightsource device that serves as the light source device of the firstembodiment of this invention.

As shown in FIG. 2, a semiconductor laser 200 is die-bonded andwire-bonded to a submount 202 (details are not shown) and mounted on aresin substrate 203 via this submount 202. The optical axis 201 of thesemiconductor laser 200 approximately coincides with the optical axis206 of a lens portion 205 a that serves as the magnifier of the secondregion 205. After the mounting of the semiconductor laser 200 on thesubmount 202, a paste-like silicone gel having a high viscosity of 30000cP (30 Pa·s) in which the scatterers are dispersed at a high density anda comparatively large amount of hardening material is blended is drippedso as to cover the submount 202 on which the semiconductor laser 200 ismounted. Then, through a gel hardening process at 150° for one hour, afirst region 204 as shown in FIG. 2 is formed. Subsequently, the deviceis sealed in the form of an eye-safe light source device with the secondregion 205 through a transfer molding process with a generalthermosetting type epoxy resin, completing the multiple scatteringoptical system constructed of the first and second regions 204 and 205.It is easy to obtain the modification example of the construction of theeye-safe light source device not only when the resin substrate 203 isemployed but also when a lead frame is employed without interposition ofthe submount.

Here is described an example of difficulty in constituting the multiplescattering optical system of this invention if a substance other thanthe gel-like or rubber-like substance is used as the base material ofthe first region 204. Although monodispersion of particles in a siliconeoil of a low viscosity is comparatively easy, it is impossible to fixthe dimensions of the first region since the silicone oil cannot behardened. Moreover, the dispersed scatterers subside, and the scatteringcharacteristic changes with a lapse of time. Moreover, if the oil isretained by using some frame, there disadvantageously occurs the outflowof the greater part of the oil during the transfer molding process forforming the second region or transubstantiation during the thermosettingprocess.

In the eye-safe light source device of this first embodiment, athickness from the surface of the resin substrate 203 to the top portionof the first region 204 can be changed in a range of from about 1 mm to4 mm. Moreover, in this eye-safe light source device, a distance fromthe surface of the resin substrate 203 to the top of a lens portion 205a of the second region 205 is set to 4.0 mm, and the radius of the lensportion 205 a is set to 2.0 mm. Particles of various material systemsdescribed later were used as the scatterers of the first region 204, themode Ds of the particle size was changed within a range of a sizeparameter q of 0.05 to 50 every produced module (eye-safe light sourcedevice), and a refractive index difference Δn between the scatterers andthe base material was changed to be from 0.02 to 1 or more. In thiscase, the size parameter q is expressed by:q=(2π,λ)·(Ds/2)(where Ds represents the particle size mode of the scatterers, and λrepresents the emission wavelength of the semiconductor laser in thesilicone gel). Moreover, a dispersion density was changed within a rangeof 0.01 vol % to 50 vol % in terms of true specific gravity. The amountof speckles of numbers of eye-safe light source devices that includedthe first region 204 of different scattering characteristics was thusevaluated.

The particle powder and the dispersion process thereof are describedhere in detail. Powders having a specified particle size distributionmode Ds within the range of the size parameter q are prepared.Typically, an aggregation (agglomerate) of particles (primary particles)that become the scatterers of which the particle size is controlled issupplied as a dry powder in the form of containing a large amount ofair. The more preferable particles as the powder to be dispersed in thefirst region in this invention are subjected to surface processingappropriate for making easy dispersion in a dispersion base material(base material of the first region) in a state in which the powder isonce scaled down in size to the primary particles in the manufacturingprocess of the powder to reduce the surface energy thereof.

When the powder is dispersed at a high density in the base material, themost preferable scatterers can be obtained typically by carrying out thetwo-step dispersion processes as follows. First of all, the powder isstiffly kneaded at a high density of 70% to 90% or more by powderpercentage by weight as a masterbatch of the scatterers. In thisprocess, the agglomerate contained in the dry powder is effectivelypulverized. Further, through the process of wetting the powder bycontinuously replacing the contained air with the base material and thekneading process of mixing and dispersing the powder into the basematerial, the primary particles can be brought into a uniformlydistributed state. For this series of processes, there is normally useda kneading machine, which has a comparatively large size and is able toapply a strong shearing force, such as a one-axis or two-axis screwextruder, kneader, homogenizer or the like.

Next, the masterbatch is kneaded (or mixed and agitated) while beingdiluted with the dispersion base material, and the scatterers at thedesired scatterer density can be obtained. In this dilution process,satisfactory scatterers that contain no agglomerate can be produced atan arbitrary density even if a comparatively simple type kneadingmachine such a kneading mixer that employs a small-sized kneader orblade, a small-sized homogenizer or bead mill, a vessel-rotating typedesktop mixer or the like. Particularly, when dilution is the mainpurpose, a high-speed vessel-rotating type mixer in which the generationof air bubbles is restrained is preferable.

The range of parameter of the powder produced for trial purposes for theeye-safe light source device covers not only the range in which thepreferable scatterers can be obtained through the aforementionedtwo-step dispersion processes but also an extremely high density rangeor an extremely small particle size range such that the formation of anagglomerate or cluster cannot be avoided due to the performance of thedisperser even in the case of the primary particle powder that hasundergone the aforementioned surface processing. By comprehensivelyevaluating these factors, the construction of the multiple scatteringoptical system that can be actually easily manufactured can beextracted.

FIG. 3A shows the relation between the full width at half maximum andthe amount of speckles obtained from the observation results of thenear-field pattern in the aforementioned various eye-safe light sourcedevices. FIGS. 3B through 3D show measurement examples and analysisexamples at the data points at which the speckles are remarkable, andFIGS. 3E through 3G show measurement examples and analysis examples atthe data points at which the speckles are sufficiently reduced. Thesenear-field patterns are obtained by observing the secondary planar lightsource formed at the interface between the first region and the secondregion by means of a CCD camera (resolving power: about 4 μm) from theoutside of the multiple scattering system and scanning the lightintensity distribution in a direction (X) approximately perpendicular tothe optical axis. Although the curve of the raw data and the curve ofthe average intensity distribution overlap with each other and lacksdistinctness, the smoother curve represents the average intensitydistribution.

Although the detail will be described later, FWHM represents the averagefull width at half maximum, and σ_(NFP) represents the amount ofspeckles based on the residual from the average intensity distributionin FIGS. 3B and 3E. Moreover, in FIGS. 3C and 3F, σ_(PAR) represents theamount of speckles based on PAR (Peak-to-Average Ratio).

The resolving power of the CCD camera used here is required to be higherthan the typical amount of flicks of the eyeball, whereas an extremelyhigh resolution is not significant for the consideration of eye safety.If a sharp speckle structure having a size of about 0.5 μm (halfwavelength) remarkably exists in the near-field pattern of the multiplescattering optical system that employs a laser element of an emissionwavelength of 1 μm, the arrangement means that optical paths mutuallyintersecting in opposite directions occupy the major part in the localregion and the optical paths eccentrically exist in a certain plane.Such the phenomenon cannot occur after passing the extremelyhigher-order multiple scattering in terms of the theory of probability.Moreover, it is impossible that a minute structure finer than the halfwavelength is generated from the coherent interference pattern itself.Therefore, it is an appropriate selection to evaluate the eye safety ofthe objective multiple scattering optical system of this invention byusing the resolving power of about 1 μm to 10 μm possessed by anordinary CCD.

In order to perform the quantitative evaluation of the speckles, σ_(PAR)(or σ_(NFP)) can be defined as a value on the vertical axis of FIG. 3A,i.e., the amount of speckles of the near-field pattern. First of all, adiscrete one-dimensional light intensity distribution I (X_(i); Y=Yj)scanned in the X-direction with a certain Y=Yj is standardized on an X-Yplane (CCD imaging plane) (1≦i≦N). Next, a smooth spatial distribution J(Xi; Y=Yj) that has undergone a smoothing process by, for example,polynomial approximation is obtained for I (Xi; Y=Yj). Further, aresidual ρ_(i)=I(X_(i); Y=Yj)−J(X_(i); Y=Yj) at each measurement pointX_(i) is obtained for all of N points with respect to this average valuecurve J(X_(i); Y=Yj) in which the speckles are virtually averaged. Thestandard deviation σ_(NFP) of the residual ρ_(i) at each measurementpoint X_(i) is expressed as follows.

$\sigma_{NFP} = \sqrt{{\frac{1}{N}{\sum\limits_{i = 1}^{N}{\rho_{i}}^{2}}} - {{\frac{1}{N}{\sum\limits_{i = 1}^{N}\rho_{i}}}}^{2}}$

It is to be noted that the expected value throughout the spatial axis(X-direction) of the residual ρi at each measurement point X_(i) iszero. Moreover, a ratio I (X_(i); Y=Yj)/J(X_(i); Y=Yj), i.e., thestandard deviation σ_(PAR) of a curve to average value PAR(Peak-to-Average Ratio) is obtained as follows.

$\sigma_{PAR} = \sqrt{{\frac{1}{N}{\sum\limits_{i = 1}^{N}{\frac{I\left( {X_{i};{Y = Y_{j}}} \right)}{J\left( {X_{i};{Y = Y_{j}}} \right)}}^{2}}} - {{\frac{1}{N}{\sum\limits_{i = 1}^{N}\frac{I\left( {X_{i};{Y = Y_{j}}} \right)}{J\left( {X_{i};{Y = Y_{j}}} \right)}}}}^{2}}$

It is to be noted that the expected value throughout the spatial axis ofPAR (X-direction) is one.

In this case, as a method for deriving the average value curve J(X_(i),Y=Yj), it is possible to average a plurality of parallel scan imagesI(X_(i); Y=Yk);k=j±1, 2, 3, . . . adjacent to the I(X_(i); Y=Yi) at eachpoint X_(i) or to use an average value of all the data points locatedwithin a specified radius from each point (X_(i), Yi) in two dimensions.Such the averaging operation is carried out by using a statisticallysufficient number of data points including at least an expansion on thelevel of the correlation size of speckles at each individual measurementpoint X_(i) and limiting the data points within a range in which theinfluence of the configuration or the size of the entire near-fieldpattern is not exerted. By carrying out this operation while scanningthroughout I(X_(i); Y=Yj), the average value curve J(X_(i); Y=Yj) isobtained.

Among the aforementioned averaging methods, particularly the method ofaveraging a plurality of peripheral data points or data rows can becomea more preferable method for an image including a discontinuous changethat cannot be expressed by a polynomial expression when there is ashadow of the wire bonding from the surface of the semiconductor laser.Moreover, it is also possible to take the ensemble average from numbersof equivalent samples. Moreover, it becomes possible to systematicallyevaluate various multiple scattering optical systems of which thedistribution forms of the average value curve J(X_(i); Y=Yj) are quitedifferent from one another by the devising of evaluating the amount ofspeckles of I(X_(i); Y=Yj) by limitation to 1/e² or higher or 1/e orhigher or within the range in which an intensity not lower than thehalfpower is possessed from the peak intensity of the average valuecurve J(X_(i); Y=Yj) or the like.

Particularly, it is preferable to assume the probability distribution ofthe amplitude of the PAR and its deviation σ_(PAR) as indexes asdescribed below as an evaluation method for reliably considering eyesafety. FIGS. 3C, 3D, 3F and 3G show the corresponding PAR's and alsotheir amplitude histograms.

According to the construction of the multiple scattering optical system,in the parameter region in which the speckles are reduced comparativelysatisfactorily, the spatial structure of the speckles is scaled down insize to a level smaller than the typical amount of flicks (several tensof micrometers), and the occurrence probability of the amplitude of PARcomes to exhibit a Gaussian distribution. Typically, PAR appears withinabout ±0.06 around the expected value of one. In FIG. 3F, PAR is withinabout 1±0.03. As is apparent from the figure, when the amount ofspeckles is evaluated by a CCD that has the finite resolving power, anupper limit value PAR_(max) at which the occurrence probability of thePAR amplitude becomes zero is easily found. Moreover, the so-calleddeviation σ_(PAR) can also be obtained directly from a PAR amplitudedistribution or by Gaussian-fitting of it.

By obtaining the PAR_(max) or the amount of speckles σ_(PAR), eye safetyis secured as follows. In detail, it is possible to consider the maximumvalue of power or energy increment per unit area due to the disturbanceof light intensity regardless of the details of the structure unit(minute spot size) of the observed residual speckles. For example, ifPAR_(max)=0.06, then the maximum value of PAR in the near-field patternbecomes about +6% also in a sample arbitrarily extracted from a lot ofmanufactured same eye-safe light source devices. Similar considerationis possible when the amount of speckles σ_(PAR) is used. According tovarious optical characteristics of the apparent light source size,emission wavelength, angular distribution of the radiant intensity andso on possessed by the eye-safe light source device, AEL (AcceptableEmission Limit) or an optical output upper limit P assuming that nospeckle exists can be determined conforming to the international safetystandard or the like. Further, by providing a margin α, the upper limitvalue P_(LIM) of the optical output of the eye-safe light source deviceis set as:P _(LIM) =P/(1+PAR _(max)+α).For example, assuming that α=4%, then the upper limit value P_(LIM) ofthe optical output is about 90% of P, and the output upper limit valueas a product specification is rendered not greater than P_(LIM).

According to the construction of the multiple scattering optical systemdisclosed in this invention, simple constituent elements andmanufacturing processes can be adopted so that the margin α becomesequal to or lower than approximately 10% anticipating thereproducibility of the manufacturing processes and a change with a lapseof time. Moreover, it is also possible to set the specification value ofthe output upper limit value and select non-defective articles by3σ_(PAR) to 6σ_(PAR) similarly to the normal process control and qualitycontrol. As a result of taking the statistics of not only theconstructions of the aforementioned multiple scattering optical systemsbut also all the constructions disclosed in this invention with regardto the relation between PAR_(max) and σ_(PAR), its average value<PAR_(max)> was about 3σ_(PAR), and its maximum value max{PAR_(max)} wasabout 5σ_(PAR).

As described above, it is reasonable enough to set a margin fromPAR_(max) and so on to design and manufacture an eye-safe semiconductorlaser module as a light source device. It becomes a good criterion totypically set σ_(PAR) to a small value of about 3% to 8% as the speckleupper limit value for securing the Class 1 level eye safety. Moreover,the relation between the amounts of speckles σ_(NFP) and σ_(PAR) has apositive correlation that depends on the overall configuration or thefull width of the near-field pattern and exhibits a similar behaviorwith respect to the parameter change of the portions of the multiplescattering optical system. Therefore, it is also possible to verify themultiple scattering optical system by the amount of speckles σ_(NFP) andcontrol the manufacturing processes.

Moreover, it is also possible to perform evaluation based on another wayof thinking for eye safety. Normally, with respect to an incoherentlight source, the apparent size is defined as the size of the regionthat includes 63% (=1−1/e) of the overall light intensity. In contrastto this, in the case of a light source including speckles, assumingthat, for example, the upper limit value of the speckle deviation atwhich the probability of including no speckle disturbance falling below1/e of the peak intensity becomes 99.9999% is σ_(LIM) (≧σ_(PAR)), thenit is possible to set an upper limit value σ_(LIM)=0.09 in considerationof the integral value of the Gaussian distribution. As described in theaforementioned several examples, the Class 1 level eye safety can besecured by performing quality control of the actual manufacturingprocesses or products by feeding the requirement for the light sourcenear-field pattern based on some models of eye safety back to thestatistical amount of PAR. Of course, when it is determined that thesafety factor is insufficient by the safety of the manufacturingprocesses and so on, it is possible to produce products that reliablyguarantee the Class 1 eye safety by redesigning the multiple scatteringoptical system for expanding the light source size so as to strictlysatisfy the margin for 6σ_(PAR) or increasing the margin a to limit theupper limit value P_(LIM) of the optical output or taking anothermeasure.

For the sake of simplicity, the data shown in FIG. 3A includes only theexamination results of the use of the single body of the scatterers ofSiO₂ (x: absolute value of refractive index difference Δn is about0.02), acrylic polymer (Δ: absolute value of refractive index differenceΔn is about 0.09), styrenic polymer (•: absolute value of refractiveindex difference Δn is about 0.19) and TiO₂ (□: absolute value ofrefractive index difference Δn is about 0.9) with silicone gel used as adispersion base material. In this case, the horizontal axis of FIG. 3Arepresents the full width at half maximum of the near-field pattern, andthe parameters, which change this, are the height of silicone gel (204shown in FIG. 2) after being hardened and the particle size, refractiveindex and dispersion density of each scatterer. As shown in FIG. 3A, asa general tendency, by increasing the height of the silicone gel andreducing the particle size or increasing the density of the scatterers,the speckles are reduced when the optical depth or the transport opticaldepth is increased. Although no detailed description is provided for thecomparison of individual data, the following matters have become clear.

With regard to the combination of the thickness in the optical axisdirection of the first region in which the transport optical depth(=geometrical dimension L/transport mean free path l_(AVE)) becomes afew times to several tens of times or, in particular, one to ten timeswith the filler from the distribution of the particle size mode Ds withrespect to an identical scatterer, a multiple scattering optical systemcapable of obtaining the Class 1 level eye safety can be constitutedwithout significantly impairing the output efficiency. Moreover, theeffect of sufficiently reducing the speckles cannot be obtained when thedispersion density is smaller than 1 vol % and conversely when thedispersion density largely exceeds 30 vol % for every scatterer.Particularly, when the dispersion density is not smaller than 30 vol %,the speckles tend to increase although the light source size itself isexpanded. Furthermore, the speckles cannot be sufficiently reduced inthe entire dispersion density range when the particle size mode Ds orits size parameter q is not smaller than 20 even in the case of a metaloxide (TiO₂ or the like) of which the refractive index difference iscomparatively large. Moreover, when the size parameter q falls belowone, there are the tendencies that the hiding power is intense and theoutput efficiency is reduced, the monodispersion itself is alsoextremely difficult and the control of the scattering characteristic isdifficult. Moreover, in the case of SiO₂ of an extremely smallrefractive index difference, it is difficult to constitute a multiplescattering optical system capable of obtaining eye safety with athickness in the optical axis direction within several millimeters.

As described above, it was found to be preferable to provide a region inwhich the size parameter q of the scatterers mainly dispersed in thefirst region is within a range of approximately 1 to 15, the asymmetryof the scattering amplitude is small and the backscattering component isnot remarkable for the construction of the multiple scattering opticalsystem of FIG. 2. Furthermore, by employing a scatterer that has arefractive index difference of about Δn≧0.15 to the base material of thefirst region and setting the size parameter q so that the product Δn·qof the refractive index difference Δn and the size parameter q fallswithin a range of approximately two to eight (particularly about three),sufficient multiple scattering was able to be generated within a shortergeometrical distance (optical path length without multiple scattering)with the asymmetry of the scattering amplitude reduced and thescattering cross-sectional are maximized.

In this case, it is needless to say that the desirable relation of therefractive index difference Δn should be satisfied for the emissionwavelength of the employed semiconductor laser when the scatterer isselected in this invention. However, there is caused no serious problemeven when the refractive index with respect to the sodium D line (about589 nm) easily available is used as a criterion for selection as thespecifications of various powder materials. The proper material as thescatterer of this invention is the one that has a bandgap wavelengthsufficiently shorter than the laser emission wavelength and does notabsorb the laser beam. When a material that does not absorb light in thevisible region (the aforementioned 589 nm) is employed in the nearinfrared region as the scatterer of this invention, there occurs nolarge error of the refractive index difference Δn in the same normaldispersion region.

Moreover, it was found to be possible to carry out a dispersion processpracticably and simply with a criterion that particularly the averagedistance between the nearest neighbor scatterers falls within twentytimes the parameter Ds for the scatterer that satisfied theaforementioned requirements. In this case, the parameter of the particlesize mode Ds of this invention should be applied to each individualparticle species in any situation in a case where the particles aremonodispersed, a case where particle species having a plurality ofparticle size distributions are dispersed in mixture, a case where asecondary particle or an agglomerate of fractal cluster or the like isgenerated, a case where these are polydispersed and so on. It ispreferable that the mode Ds is obtained in consideration of the outsidediameter itself of the agglomerate and the size parameter qcorresponding to it satisfies the aforementioned range of approximately1 to 15 for a dispersion system including an agglomerate that has acertain outside diameter but internally has a gap. In the case of theagglomerate of the same outside diameter size, the one that issufficiently smaller than the wavelength and is constructed of primaryparticles of a smaller particle size is rather able to obtain asatisfactory result for speckle reduction because of a scarce appearanceof a minute structure in the dispersion amplitude of the agglomerate.

Moreover, the dispersion density by the volume ratio does not alwaysbecome a complete definition. This corresponds to a case wheremonodispersion to the base material (gel or rubber or elastomer) is hardto obtain and an agglomerate of secondary particles is generated, a casewhere the influence of the amount of oil absorption is large or thelike. With regard to the former case of the particles that agglomerateas secondary particles, it is acceptable to consider that the relationof the average nearest neighbor distance is the desirable condition asit is if the outside diameter is in the desirable range betweenagglomerates. On the other hand, with regard to the latter case of thelarge influence of oil absorption, it is possible to constitute asatisfactory multiple scattering region so long as the volume ratioconverted by using the specific gravity (true specific gravity) of thescatterer (crystal) itself after a sufficient dispersion process (aftervolume reduction almost disappears) is within a range of approximately 1vol % to 30 vol %. When a dispersion charge amount is determined by theapparent specific gravity (apparent specific gravity including air inthe gap portion) ordinarily used as the specifications of the drypowder, there occurs an error with respect to the estimated value of theaverage nearest neighbor distance due to the true specific gravity.However, it is not difficult to confirm that the distance betweenscatterers is within about 20 time the average particle size mode Ds byactual observation with a confocal laser microscope or the like afterdispersion and the hardening processes. Therefore, it is possible tofind the desirable dispersion density by adjusting the manufacturingprocesses. It is needless to say that advertency and devising in theactual dispersion process like the deaerating process of air bubbles andso on sometimes become important in order to control the scatteringcharacteristic of the first region.

Here is described another example of difficulties in constituting themultiple scattering optical system of this invention when a substanceother than the gel-like or the rubber-like substance is used as the basematerial of the first region. For example, it is also possible to useepoxy resin as the base material. In comparison with silicone-based geland elastomer, the dispersion of particles is comparatively easy,whereas the refractive index with respect to the sodium D line generallyhas a high value of not smaller than 1.53. Therefore, when the absolutevalue of the refractive index difference Δn becomes equal to or smallerthan about 0.05 also with respect to either acrylic or styrenicparticles taken as the general organic particles among the variousscatterers previously enumerated. When epoxy resin is applied to thebase material of the first region, the desirable scatteringcharacteristic cannot be obtained due to a shortage of refractive indexdifference (on the same level as SiO₂ of FIG. 3A). On the other hand,the organic particles can easily be reduced in cost and increased inparticle size, and these organic particles are extremely effective inthe multiple scattering optical system of this invention in which ascatterer of a comparatively large diameter described later iseffective. However, it is extremely inefficient in the actualmanufacturing processes to selectively use a plurality of base materialsin the stage in which the construction of the multiple scatteringoptical system is selected or optimized, and it is difficult to say thatthe material is most preferable as the base material of the first regionof the multiple scattering optical system of this invention.

The construction of the eye-safe light source device shown in FIG. 2 ispreferably used in a case where a combination of the base material thatpermits the high-density dispersion of the scatterers of a comparativelysmall particle size and a kneading device can be used or in a case wherean eye-safe light source device that is less demanded to reduce the sizeis manufactured. On the other hand, according to the construction of theeye-safe light source device shown in FIG. 2, although the manufacturingprocesses are simple, it is difficult to control the thickness in theoptical axis direction of the first region and the size of the secondaryplanar light source, and it is comparatively difficult to particularlyefficiently expand the size of the secondary planar light source.Moreover, if it is tried to improve the flowability of the base materialthat constitutes the first region and facilitate the handling afterhigh-density dispersion, it is difficult to control the configurationand the scatterer density of the first region, and this possibly causesa problem of manufacturing yield. Therefore, it is sometimes the casewhere the aforementioned construction is hard to adopt when an eye-safelight source device, in which the required specification of the opticaloutput is high and a large margin cannot be taken for AEL (AcceptableEmission Limit), is designed and manufactured. Accordingly, an eye-safelight source device that solves such a problem will be described in thefollowing second embodiment.

SECOND EMBODIMENT

FIG. 4 is a sectional view showing the construction of an eye-safe lightsource device as the light source device of the second embodiment ofthis invention. In FIG. 4, the same components as those of the eye-safelight source device shown in FIG. 2 of the first embodiment are denotedby the same reference numerals.

As shown in FIG. 4, a semiconductor laser 200 is vertically placed on aresin substrate 203 via a submount 202, and thereafter, a cylinder 400(inside diameter: 1.1 mm, height h=2 mm) is fixed with a silver paste soas to surround the submount 202 on which the semiconductor laser 200 ismounted. As described above, there is formed a recess portion 210 inwhich the inner surface of the cylinder 400 is served as a wall surfaceand the surface of the resin substrate 203 is served as a bottomsurface. A metal layer 401 is formed by Au plating on the innerperipheral surface of the cylinder 400. An Au plating wiring pattern 402is formed on the resin substrate 203. A principal portion of theoutermost surface of the recess portion 210 is provided by a metal layerconstructed of the metal layer 401 of the cylinder 400 and the Auplating wiring pattern 402 on the resin substrate 203.

Furthermore, scatterers with the base material made of silicone gel ofthe first region 204 are injected into the recess portion 210 and madeto have a fixed configuration. Through a subsequent gel hardeningprocess, the configuration and the scattering characteristic aresatisfactorily maintained. As schematically shown in FIG. 4, a laserbeam, which travels in the optical axis direction 206 of the lensportion 205 a that serves as the magnifier of the second region 205while being diffused by multiple scattering, is reflected upon reachingthe wall surface of the recess portion 210, and scattered light travelstoward the second region 205 as a whole while being confined in thefirst region 204. The principal portion of the outermost surface of therecess portion 210 means a plane brought in contact with thisthree-dimensional diffusion region. With this construction, the openingof the cylinder 400 becomes a light dispersion port from the firstregion 204, and a laser beam, which has undergone sufficient multiplescattering, is guided to the second region 205. As a result, a secondaryplanar light source, which has a configuration similar to the opening ofthe recess portion 210, is formed at the interface between the firstregion 204 and the second region 205 existing in the vicinity of theopening of the cylinder 400. The multiple scattering optical system isconstituted of the first region 204 and the second region 205.

FIG. 5A shows the results of evaluating the amount of speckles σ_(PAR)of the near-field pattern with respect to a number of eye-safe lightsource devices obtained by variously changing the height h of thecylinder 400 and changing the scatterer parameters and density in a widerange similarly to FIG. 3A in the construction of FIG. 4. Moreover,FIGS. 5B, 5C and 5D show a satisfactory near-field pattern, PAR and PARhistogram. The two-dimensional configuration of the near-field patternhas a sharp cutoff skirt portion reflecting the cross-sectionalconfiguration of the cylinder 400 (shown in FIG. 4). Therefore, thelight source size can be clearly defined, and the reproducibility in themanufacturing stage is drastically improved. Therefore, an eye-safelight source device can be produced at low cost with high yield.Moreover, if the geometrical length of the first region has almost thesame value as that of the light source device shown in FIG. 2, itbecomes possible to more effectively reduce the speckles by virtue ofthe arrangement that the multiple scattering region is surrounded by therecess portion 210 of which the outermost surface is provided by themetal layer.

In FIG. 5A, dissimilarly from FIG. 3A, the transport optical depthL/l_(AVE) is plotted on the horizontal axis. In the aforementionedconstruction, the thickness L in the direction of the optical axis 206of the first region 204 shown in FIG. 4 can be clearly measured, andtherefore, a more comprehensive evaluation can be achieved as shown inFIG. 5A. It is possible to obtain a scattering cross section and anasymmetry factor g in accordance with the Mie scattering theory from thescatterer parameters and calculate a transport mean free path l_(AVE)from the dispersion density. In FIG. 5A, the feature also seen in FIG.3A was observed as a more general tendency without depending on the typeof the scatterer. That is, if the transport optical depth exceeds one inthe first region, which is the multiple scattering region that isbrought in contact with the high-power semiconductor laser and isprovided so as to surround this, it becomes possible to obtain anear-field pattern in which the speckles are reduced to the level atwhich the Class 1 level eye safety is satisfied.

However, if the rightward-sloping tendency is pursued, there are alsoconstraints on the range in which the parameters (refractive indexdifference and particle size) of the scatterer itself can be changed,and eventually, the dispersion density cannot help being increased to,for example, 30 Vol % to 50 Vol %. The tendency that the speckle ratherincreased in the high-density region with any of the scatterers wasevidently observed. If the dispersion gel in this case is observed by anoptical microscope, the greater part of the scatterers is often found tobe brought in contact with one another into a cluster. Although theupper limit value of the transport optical depth L/l_(AVE) at which thespeckles increase again differed depending on the scatteringcross-sectional area (i.e., mainly the refractive index difference Δn)standardized by the geometrical cross-sectional area of each scatterer,it was found to be difficult to stably obtain a multiple scatteringregion such that the multiple scattering is preferably generated severaltens of times or more times with respect to the dimensions of theportions supposed by this invention.

The scatterer species preferably applied to the construction of anoptical system in which the geometric optical path length can be clearlydefined as the multiple scattering optical system of this invention asin, particularly FIG. 1, 2 or 4 is not required to be limited to thoseshown in FIGS. 3A and 5A. For example, in the case where the siliconegel (refractive index to the sodium D line is about 1.40) is used as thebase material, CeO₂ and ZrO₂ (refractive index: 2.3) are also suitablyused in addition to the styrenic crosslinked polymer (refractive index:1.59) and TiO₂ (refractive index: 2.6). Alternatively, metal oxides suchas ZnO (refractive index: 2.0), Al₂O₃ (refractive index: 1.77) and thelike, hydroxides such as Al(OH)₃ (refractive index: 1.6) and the like orvarious glass beads (refractive index: about 1.5 to 1.6) can also bepreferably employed. Furthermore, it is possible to employ hollowparticles (refractive index: 1.0) whose outer shell is provided by avariety of materials, calcium carbonate (refractive index: 1.6) used forpigments, a variety of other minerals among coloring agents so long asthe minerals do not absorb the laser beam. Moreover, an ultravioletcuring type resin material, which generates perfectly spherical gaps andair bubbles (refractive index: 1.0) according to the curing condition,can be enumerated as a special example of the base material other thanthe silicone gel. Moreover, the shapes of the above-mentioned variousparticles are not always required to be perfectly spherical, and amultiple scattering optical system can be roughly satisfactorilyconstituted also by obtaining the mode Ds by averaging each dimension(size).

In the eye-safe light source device described with reference to FIG. 2of the first embodiment and FIG. 4 of this second embodiment, thesemiconductor laser is provided by the one that has anInGaAs/AlGaAs-based structure of an emission wavelength of 890 nm. Thatis, the active layer well layer of an ordinary 780-nm band AlGaAs-basedlaser for the CD-R/RW is changed to an InGaAs strained quantum welllayer. It was discovered that a significant difference was generatedeven in the CW (Continuous Wave) operation in the wavelength spectrumexpressed by the stimulated emission after multiple scattering and thewavelength spectrum measured with the single body of the semiconductorlaser in the various module states shown in FIG. 3A or 5A. Particularly,an inter-longitudinal-mode contention state during high-power operationwas remarkably observed in comparison with the case of a single laser inthe module that had undergone comparatively high density dispersioncapable of obtaining a satisfactory coherency reducing effect of avolume ratio of about 1 vol % to 30 vol %. In the construction of thisinvention, the first region of the multiple scattering optical system isformed so as to be adjacent to the semiconductor laser and to surroundthe laser. Therefore, the coherent backscattering peak componentgenerated by the multiple scattering is fed as the so-called returnlight back to the output end surface of the semiconductor laser. It ispresumed that the effect of increasing the spectral linewidth and theeffect of inducing mode contention are generated by this return light(described later), and this can be regarded as the remarkable feature ofthe multiple scattering optical system of this invention.

The arrangement that the first region is located adjacent to thesemiconductor laser is described here. The first region is not alwaysrequired to be brought in direct contact with the semiconductor laser,and a gap region or a layer that has no scattering function of the firstregion may be placed between the semiconductor laser and the firstregion. For example, when the wettability of the semiconductor laserwith respect to the base material of the first region is not good, a gapportion is generated at the interface. Moreover, a gap is generatedbetween the semiconductor laser and the first region also in the casewhere a single-layer or multi-layer dielectric coating on the endsurface of the semiconductor laser is formed thick to a thickness of notabout a few times the normal wavelength but several tens of times ormore times the wavelength. Moreover, it is possibly desired tointentionally avoid the contact between the semiconductor laser and thefirst region and avoid the close adhesion of the scatterers to the laserend surface. In such a case, the first region can sufficiently obtainits various operations so long as the end surface of the semiconductorlaser chip and the first region are located adjacent to each othertypically within a distance of about 1 mm. Otherwise, when it isdesirable that the coherent backscattering peak is fed back to the laserlight output end surface and arranged adjacently to an extent that thelight acts as a return light, the action of the return light can beobtained so long as the single surface of the semiconductor laser chipand the first region are located adjacent to each other at a distancewithin a range of several micrometers to several hundreds ofmicrometers. Otherwise, it is acceptable to interpose a regioncontaining comparatively large-size powder that individually behavesgeometrically optically as a minute micro halfmirror or a microlensbetween the semiconductor laser and the first region, dissimilarly tothe scatterers of the first region. That is, the optical system can beconstituted so that the luminous flux from the laser is spatiallyexpanded more effectively and the luminous fluxes divided in the firstregion subsequently undergoes multiple scattering. According to theconstruction, it becomes easier to reduce the speckles and expand theapparent light source size. As in the plurality of examples describedhere, the semiconductor laser and the first region are not alwaysrequired to be provided in direct contact with each other.

In the eye-safe light source device of the construction of FIG. 2 or 4,the total thickness of the multiple scattering optical system, i.e., thedimension in the optical axis direction is typically required to be notsmaller than about 4 mm. It is of course possible to reduce the totalthickness of the optical system by providing a seating hole on the resinsubstrate 203 in FIG. 2 and obtaining a function similar to that of therecess portion 210 of FIG. 4. Moreover, it is possible to use acomposite recess portion given by a combination of a seating hole andthe aforementioned cylinder.

However, the structural example of FIG. 1, 2 or 4 does not haveremarkable operation for reducing the thickness of the multiplescattering optical system. Therefore, it is possibly the case where theapparent light source size cannot be sufficiently expanded and the upperlimit value of the optical output cannot help being limited when anextremely small-sized eye-safe light source device is desired to beobtained. Accordingly, an eye-safe light source device that solves suchproblems and more effectively functions will be described in thefollowing third embodiment.

THIRD EMBODIMENT

FIG. 6A is a sectional view showing the construction of an eye-safelight source device that is a third embodiment of the light sourcedevice of this invention. FIG. 6B is a schematic view showing theoptical path of a semiconductor laser. FIG. 6C is a graph showing therelative light intensity distribution of a near-field pattern.

A semiconductor laser 600 employed in the eye-safe light source deviceof this third embodiment was provided by one whose both end surfaces arehalf-wavelength coated and had a chip thickness of 100 μm, a chip widthof 230 μm, a ridge stripe width of 2.5 μm and a cavity length of 500 μm.In this semiconductor laser 600, an InGaAs MQW active layer andAlGaAs-based barrier/guide/cladding layers as used in ordinary 780-nmband lasers are adjusted so that the emission wavelength becomes 890 nm.The semiconductor laser is quite the same as the semiconductor lasersdescribed with reference to FIGS. 2 through 5A, 5B, 5C and 5D of thefirst and second embodiments except for the point that an optical outputis obtained from both end surfaces by changing the end surface coatingfrom AR/HR to half-wavelength coating. The semiconductor laser of thistype, which provides a threshold current of about 10 mA and a COD(Catastrophic Optical Damage) level exceeding 250 mW, is thus a veryadvantageous device in increasing the output of the transmission meansin a transceiver of a low-cost high-speed wireless optical communicationsystem that employs Si as a detector (photodetector).

In FIG. 6A, a recess portion 610, which has a flat portion (bottomsurface) of a depth of 350 μm and a cavity direction length of 600 μmand a wall surface (inclined surface) of an inclination angle of about50°, is formed on the upper surface of a resin substrate 603. A wiringpattern 612 is formed by Au plating on the resin substrate 603 and theoutermost surface of the recess portion 610. The lower surface of thesemiconductor laser 600 is die-bonded to the bottom surface (wiringpattern 612) of the recess portion 610 by using a silver paste, and theupper surface of the semiconductor laser 600 is wire-bonded (by wire602) to another wiring pattern 608.

In the above-mentioned eye-safe light source device, for example, astyrenic crosslinked particles that have an average particle size of 0.8μm (q=4 and g=0.7 in silicone gel) and a particle size accuracy CV valueof 50% is employed as a preferable scatterer. A thermosetting typesilicone gel in which the styrenic crosslinked particles dispersed asscatterers at a high density of 15 wt % can be employed (refractiveindex difference Δn is approximately 0.2). The details of variousscatterers will be described later.

The silicone gel has a principal ingredient of dimethyl polysiloxane andsingly has a comparatively high flowability of a viscosity of 2000 mPa·s(2000 cP). Such the silicone gel main material is subjected to adispersion process of the scatterers by a kneader and thereafter to adeaerating process in mixture with a hardening agent and injected intothe recess portion 610 with a flowability of about 5000 mPa·smaintained. The high-density dispersion gel injected into the recessportion 610 is subjected to a thermosetting process of at least 180° C.for one hour with its configuration defined and maintained by the recessportion 610 and becomes the first region 604 of the multiple scatteringoptical system of this invention. Subsequently, by transfer-molding athermosetting type epoxy resin that contains no scatterer, a secondregion 605 that is put in contact with the first region 604 and reachesa free space is formed, and the entire upper surface of the resinsubstrate 603 is integrally sealed. The second region 605 includes atleast a lens portion 605 a that serves as a magnifier having an opticalaxis 606 constituted by using a portion of a spherical surface of aradius of 1.1 mm. A geometrical distance from the bottom surface of therecess portion 610 to the top of the lens portion 605 a is 1.65 mm. Amultiple scattering optical system is constituted of the first region604 and the second region 605.

In the eye-safe light source device of the above-mentioned construction,the direction of the optical axis 601 of the outgoing light from bothend surfaces of the semiconductor laser 600 is converted so as to directapproximately the same direction as that of the optical axis 606 of thelens portion 605 a of the second region 605 until reaching the interfacebetween the first region 604 and the second region 605.

Furthermore, it is also preferable to combine the construction of theeye-safe light source device of FIG. 6A of this third embodiment withthe construction of the eye-safe light source device of FIG. 4 of thesecond embodiment. Two types of the construction of the eye-safe lightsource device of such combination are shown in FIGS. 7A and 7C.

In the eye-safe light source device shown in FIG. 7A, atruncated-cone-shaped recess portion 730, which extends upwardly of theresin substrate 703, is provided, and a semiconductor laser 700 isarranged inside the recess portion 730. An Au plating wiring pattern 712is formed on the resin substrate 703 and the recess portion 730, and alower electrode (not shown) of the semiconductor laser 700 iselectrically connected onto the Au plating wiring pattern 712 of therecess portion 730. On the other hand, the upper electrode (not shown)of the semiconductor laser 700 is electrically connected to an electrode713 provided on the resin substrate 703 and in the vicinity of therecess portion 730 via a wire 702. Further, a cylinder 710 is arrangedon the resin substrate 703 so as to surround the recess portion 730. Thecylinder 710 is fixed on the resin substrate 703 with a silver paste.Further, by forming a metal layer 711 on the inner wall of the cylinder710 by Ag plating, the inner surface is served as a reflective surface.

A composite recess portion is defined by the wall surface and the bottomsurface of the recess portion 730 and the inner wall surface of thecylinder 710. A first region 704 is formed by filling this compositerecess portion with the same high density dispersion gel as that of theeye-safe light source device of FIG. 6A. Then, a second region 705molded with epoxy resin is formed on the resin substrate 703. The secondregion 705 has a lens portion 705 a that serves as a magnifier. In thiscase, output light from one end surface of the semiconductor laser 700provided with the AR/HR coating is utilized, and the wall surfaces ofthe recess portion 730 are opposed at an angle of 45° in correspondencewith this.

On the other hand, dissimilarly from the eye-safe light source deviceshown in FIG. 7A, the eye-safe light source device shown in FIG. 7Cemploys a cylinder 720 that has a diameter smaller than that of thecylinder 710 and forms a first region 714 by filling a composite recessportion formed of the wall surface and the bottom surface of the recessportion 730 and the inner wall surface of the cylinder 720 with asimilar high-density dispersion gel. Moreover, the electrode 713 islocated outside the first region 704, and part of the wire 702 connectedto this electrode 713 is located in the second region 705. A metal layer721 is formed on the inner wall of the cylinder 720 by Ag plating.According to the construction of the eye-safe light source device shownin this FIG. 7C, although wire peeling or a defect of short-circuit withthe cylinder 720 tends to occur during the transfer-molding, theflatness or symmetricity of the near-field pattern is improved incomparison with FIG. 7A (see FIGS. 7B and 7D). The other constituentelements are the same as those of FIG. 6A.

A structural point that should be noted and its remarkable effects aredescribed here. In the eye-safe light source device of FIG. 6A or 7C,the wires 602 and 702 are extended to the inside of the epoxy resin ofthe second regions 605 and 705. With this construction, if the moldedportions (second regions 605 and 705) damage and peel off during theoperation of the eye-safe light source device, the wires 602 and 702 arepeeled off together with the molded portions and broken. Consequently,the electrified circuit of the semiconductor laser 600 or 700 enters anopen state. That is, in the case of the eye-safe light source device ofthe specifications that satisfy the safety standard by expanding thelight source diameter by the lens gain of the molded portion, i.e., inthe case of the eye-safe light source device of which the light sourcesize does not satisfy the safety standard in a state in which the moldedportion is peeled off, the electrification of the semiconductor lasers600 and 700 is interrupted, never exposing the user to a perilous state.It is to be noted that the above-mentioned peril does not always occurin all the eye-safe light source devices of the design specificationsthat relies on the lens gain as described above. This is because theradiant intensity distribution of the scattered light from the firstregions 604 and 704 almost becomes complete Lambertian in the absence ofthe mold lens, and the radiation angle is widened to totally reduce theradiant intensity per unit solid angle. However, the fact that aneye-safe light source device that secures sufficient safety (apparentlight source size) even with no molded portion is desirable remainsunchanged.

Furthermore, as a remarkable feature of the eye-safe light source deviceof FIG. 6A or 7C, the outermost surface metal layer of the recessportion 610 or the outermost surface metal layer 721 of the recessportion 730 is continuous within the range of the expansion of scatteredlight. On the other hand, for example, in FIG. 7A, an FR4 substrate 703is exposed between the outermost surface metal layer of a seating holeportion and the inner wall metal layer 711 of the cylinder 710 at thecomposite recess portion 704 and brought in contact with the firstregion 704 particularly inside the principal region in which the spatialdistribution of scattered light is comparatively intense as shown in thefigure. Therefore, in FIG. 7A, when the first region is constituted byusing scatterers that have a comparatively small asymmetry factor g of,for example, not greater than about 0.7 or when the geometrical opticalpath length is comparatively long even in the case of an arbitraryasymmetry factor g or in a similar case, the leakage quantity of thescattered light toward the resin substrate 703 side often becomesunignorable.

Therefore, as shown in FIG. 6A or 7C, a higher light output efficiencycan be stably obtained as a light source device by virtue of theprovision of the continuous outermost surface metal layer of theprincipal portion to construct a single or composite recess portion. Theprovision of the continuous principal metal layers is not always thestructural requirement that coincides with the arrangement that thesubstrate side wire bonding point 608 or 713 is located outside thefirst region, i.e., the aforementioned feature that the wire is extendedin the second region. However, in general, it is needless to say thatthe construction in which the wire is extended in the second region moreeasily obtains the continuity of the metal layers.

Moreover, in FIGS. 7A and 7C, the cross-sectional configuration of thecylinders 710 and 720 forming the aforementioned composite recessportion is not always required to be circular. A columnar or conicalconfiguration is most preferable in simplifying the manufacturingprocesses and reducing the cost. However, polygons inclusive of a squareand a rectangle is acceptable, and the configuration should preferablyis symmetric with respect to the axis of the semiconductor laser cavityfrom the viewpoint of the uniformity of the near-field pattern and alsothe far-field pattern. Moreover, as is apparent from FIG. 4 of thesecond embodiment and FIGS. 6A, 7A and 7C of this third embodiment, theoverall construction of the recess portion has arbitrariness. That is,it is desirable that the entire recess portion has a configurationhaving an opening toward the second region, and the outermost surfacebrought in contact with the principal portion of the scattering regionis provided by a metal layer.

As in the construction of FIGS. 6A, 7A and 7C, the inclination angle ofthe principal wall surface of the recess portion 610, 730 is not alwaysrequired to be set at 45° also in the case of the multiple scatteringoptical system in which the optical axis of the outgoing beam of thesemiconductor laser 600, 700 roughly perpendicularly intersects theoptical axis of the lens portion 605 a, 705 a of the second region 605,705. That is, it is not required to provide a complete geometricaloptical design since the first regions 604 and 704 receive extremelymany times of scattering.

Moreover, when the optical output from one end surface of thesemiconductor laser 700 is mainly utilized as in the construction ofFIGS. 7A and 7C, assuming that no scatterer exists, then it ispreferable to shift the center axes of the recess portion 730 and thelens portion 705 a of the second region 705 so that the intersection ofthe interface between the first region 704 and the second region 705 andthe optical axis 701 of the laser beam passes through the center pointof the secondary planar light source. Moreover, it is needless to saythat the peak center of the light intensity distribution on theinterface between the first region 704, 714 and the second region 705should preferably be arranged on the optical axis 706 of the secondregion 705.

Moreover, in the construction in which the optical outputs from both endsurfaces of the semiconductor laser 600 are utilized as in, for example,the eye-safe light source device shown in FIG. 6A, there is a possiblemodification such that the inclination angle of the wall surface of therecess portion 610 is set large or the inclination angle of the wallsurface of the recess portion 610 is conversely set small when thecavity of the semiconductor laser 600 is relatively short. That is,assuming that no scatterer exists in the first region, when the opticalaxis of the laser beam exists while being multiply divided, it is ratherpreferable to provide a design of a symmetric and uniform distributionwith respect to the center point of the secondary planar light source.Thus improving the flatness of the light intensity distribution of thenear-field pattern (uniformity of the averaged light intensitydistribution) has the operation of directly reducing the spatialcoherency and is also effective for speckle reduction.

FIG. 8 shows the results of comprehensively evaluating the near-fieldpattern in many eye-safe light source devices in which the variousscatterers described with reference to FIG. 3A are dispersed in varioussizes at various densities in the silicone gel with regard to theconstruction of the eye-safe light source device shown in FIGS. 6A, 7Aand 7C. The horizontal axis of FIG. 8 represents a transport opticaldepth L/l_(AVE) that is a value obtained by dividing a geometricaldistance L (see FIG. 6B) of the length of the optical axis of the laserbeam passing through the first region when it is virtually assumed thatno scatterer exists, by the transport mean free path l_(AVE) in thefirst region. The vertical axis of FIG. 8 represents the amount ofspeckles σ_(PAR) of the near-field pattern. The satisfactory relativelight intensity distributions of the near-field patterns of theconstructions shown in FIGS. 6A, 7A and 7C are shown in FIGS. 6C, 7B and7D, respectively.

It is evident from comparison of FIG. 8 with FIG. 3A or 5A that thevalue of the amount of speckles σ_(PAR) has been totally reduced.Moreover, according to the constructions of FIGS. 6A, 7A and 7C, itbecomes possible to reduce the total thickness of the optical system byeffectively increasing the scattering frequency by virtue of thearrangement that the multiple scattering region is surrounded by therecess portion of which the outermost surface is provided by a metallayer even with the same geometrical length. Particularly, there wasdistinctly confirmed a tendency that the speckles were possiblyincreased when the transport optical depth exceeded several tens oftimes by simple conversion due to the increase in the thickness of thefirst region particularly in the constructions of FIGS. 7A and 7C, thetendency being similar to FIG. 5A of the second embodiment.

Moreover, it was found that a metal oxide such as TiO₂, styrenic polymerand so on, in which the size parameter q of the particle sizedistribution mode Ds was within the range of approximately 1 to 15 andwhich could take a comparatively large refractive index difference Δn(Δn≧0.15) with respect to the silicone gel were very preferable as ascattering material for the construction of the recess portion in FIG.6A or FIGS. 7A and 7C, and this tendency is almost similar to FIG. 5A ofthe second embodiment.

A preferable scattering characteristic was obtained by dispersing thesescatterers at a density of typically 0.5 vol % to 30 vol % and morepreferably 1 vol % to 15 vol %. The dispersion gel that had thepreferable scattering characteristic was observed by an opticalmicroscope using the amount of speckles σ_(PAR)≦10⁻¹ as a criterion, andit was discovered that the scatterers were roughly uniformly distributedand an average value <R> of the nearest neighbor distance consistentlysatisfied the relation of <R>≦20Ds. Moreover, the amount of specklesσ_(PAR) abruptly rose at an average value <R>≧30Ds. Conversely, therewas distinctly observed the tendency that the speckle started toincrease when the scatterer density was typically increased to 30 vol %or higher. In the dispersion gel in this state, there was often observedthe case where a cluster of not smaller than 10 μm was formed and theratio of a sparse space was extremely increased.

As a new effect of the construction of FIGS. 6A, 7A and 7C, as isevident from the relative light intensity distribution of the near-fieldpattern of FIGS. 6C, 7B and 7D, the flatness of the entire near-fieldpattern was improved. As previously described, the aforementionedcharacteristic produces great effects (described later) since it exertsa direct influence on speckle reduction, narrows the width of thefar-field pattern and restrains the useless skirt component trailing.Furthermore, the expansion of the two-dimensional light source formed atthe interface between the first region and the second region had aconfiguration approximately similar to that of the opening of the recessportion, and therefore, an eye-safe light source device that was able todistinctly define the light source size was made manufacturable with areliable margin and high yield.

Moreover, in comparison with the construction of the eye-safe lightsource device shown in FIG. 4 of the second embodiment, there are theeffects that the geometrical length L is extended by the optical axisconversion by the recess portion wall surface that has an inclinedsurface as shown in FIG. 6B and light efficiently spreads in the firstregion as a consequence of the irregular reflection on the roughenedsurface of the metal layer of the wall surface of the recess portion.Therefore, in comparison with the construction of the eye-safe lightsource device of FIG. 2 of the first embodiment and the eye-safe lightsource device of FIG. 4 of the second embodiment, the scatterer densitycan be reduced, and the dispersion process becomes easy. Moreover, byproviding a degree of freedom in the design of the first region as inFIGS. 7A and 7C, the permissible range with respect to the particle sizedistribution of the scatterers was able to be expanded, and themanufacturing yield of the eye-safe light source device was able to beremarkably improved. Particularly, even in the case of the acryliccrosslinked polymer particles that had a comparatively small refractiveindex difference Δn of 0.09 with respect to the base material, adesirable scattering characteristic was sometimes found in acomparatively high density range of about 5 vol % to 20 vol %. Such theacrylic particles can be comparatively easily reduced in the particlesize among the organic particles, and this is preferable in terms ofcost.

The far-field pattern will be described next. In contrast to theconstruction of FIG. 6A, a distance Hc from the interface between thefirst region 604 and the second region 605 to the top of the lensportion 605 a as the magnifier of the second region 605 was changed, andthe radius R of the lens portion 605 a of the second region 605 waschanged from 1.0 mm to 1.5 mm. Moreover, by variously changing theheight h of the cylinders 710 and 720 of FIGS. 7A and 7C in theconstruction of FIGS. 7A and 7C, a distance Hc from the interfacebetween the first region 704, 714 and the second region 705 to the topof the lens portion 705 a as the magnifier of the second region 705 waschanged, and the radius R of the lens portion 705 a of the second region705 was changed between 1.0 mm and 1.5 mm inclusive. Further, accordingto the construction of FIG. 6A or FIGS. 7A and 7C, the diameter of thesecondary planar light source was also changed within a range of about0.5 mm to 3 mm. The results of evaluating the full width at half maximumof the far-field pattern and the amount of speckles σ_(PAR) regardingthe various modules are shown in FIG. 9A. Although the amount ofspeckles σ_(PAR) can be defined regarding the far-field pattern by amethod similar to that of the near-field pattern, it is required to payattention to FOV (viewing angle) of the measurement system in order toperform consistent evaluations. In this case, a specification slightlystricter than IrDA was assumed, and the radiant intensity distribution(FFP) was measured by setting a solid angle of the photodetectingportion of a diameter of 1 mm viewed from a distance of 1 m as ameasurement step and fixing the resolving power higher than this.

According to the construction of the eye-safe light source device ofFIG. 6A, the radius of the spherical lens portion 605 a is R=1.1 mm, andthe refractive index n of epoxy resin of the material is about 1.5.Therefore, the focal distance f is expressed by:f=R·n/(n−1)and the focal point appears at a distance of approximately 3.3 mm on theresin substrate 603 side away from the top of the lens portion 605 a.Since the distance Hc from the top of the lens portion 605 a to theinterface between the first region 604 and the second region 605 isabout 1.3 mm, Hc/f of the horizontal axis is 0.39. By reducing this Hc/fand arranging the focal position deeper on the resin substrate 603 side,the radiant intensity distribution (FFP) is narrowed. In accordance withthis, the amount of speckles SPAR of the far-field pattern is alsoincreased. There was found the tendency that the radiant intensitydistribution (FFP) started to spread again and the amount of speckleswas saturated when the secondary planar light source was locatedexcessively closer to the lens top. In this case, as is evident fromFIGS. 9B and 9C, it can be understood that the skirt trailing of theradiant intensity scarcely occurs in a wide-angle region outside thehalf-value angle both when the full width at half maximum iscomparatively narrow (about 20 degrees on FIG. 9B side) and when it iscomparatively wide (about 60 degrees on FIG. 9C side), and it can beunderstood that a radiant intensity distribution (FFP) resembling arectangle that cannot be expressed by the generalized Lambertian (n-thpower of cosine) is obtained.

In the greater part of the region of the data shown in FIG. 9A, theamount of speckles σ_(PAR) was reduced in inverse proportion to almostthe second power of the full width at half maximum of the radiantintensity distribution (FFP). This is attributed to the fact that thespatial coherency still remains in the near-field pattern when locallyviewed (on the order of the size of the scatterer) although the globalcoherency of the secondary planar light source is almost lost as awhole. During the process in which the final scattered light from eachsurface element of the secondary planar light source is radiated at aminute angle with the mutual correlation possessed and converted fromthe near-field pattern into the far-field pattern in the second region,the probability of re-intersection at a minute angle cannot be renderedzero. In the second region constituted so as to obtain a narrow fullwidth at half maximum, the probability of intersection is relativelyincreased, and an extremely large number of speckle patterns thatindividually have no correlation are overlapped with one another in thefar-field pattern. Therefore, it is also difficult to make the specklescompletely disappear although speckles for extremely reducing theradiant intensity are not generated. Moreover, if the radiation angle offull width at half maximum exceeds 100°, then the correlation betweenthe amount of speckles and the half-value angle is collapsed, and auseless skirt component trailing outwardly of the half-value angle comesto occur in the radiant intensity distribution (FFP) (not shown). Withregard to Hc/f of this region, it is difficult to constitute the opticalsystem so as to guide all the quantity of light to the second region,and the optical output or efficiency is also reduced.

However, by constituting the second region of the multiple scatteringoptical system as described above to reduce the probability of theoccurrence of the aforementioned intersection at a low angle, the amountof the far-field pattern speckles was able to be restrained to anonproblematic value at least in the range of full width at half maximumof a radiation angle of about 20° to 60° practically important forwireless optical communication uses (σ_(PAR)<<10⁻¹). In order to obtaina narrower radiant intensity distribution (FFP), it is proper to providethe second region with, for example, an aspherical lens and arrange theaspherical lens so that the lens operates as a magnifier for thesecondary planar light source.

The hiding power, which can become a common problem in the first regionsshown in FIG. 4 of the second embodiment and FIGS. 6A, 7A and 7C of thisthird embodiment, is described here. If the first region is constitutedby dispersing scatterers that have a high refractive index difference(refractive index difference Δn>0.5) of a comparatively small diameter(size parameter q is about one to three) in the desirable range of theconstructions shown in FIGS. 4 and 6A and FIGS. 7A and 7C at a densityof, for example, not smaller than 20 vol %, then there sometimes occursthe case of a module of which the total quantity of output light fallsshort of the half of the original output of the semiconductor laseralthough the amount of speckles σ_(PAR) can be extremely reduced to anextremely small level (<10⁻²). This is ascribed to a plurality ofcauses, which principally include the two principal causes of the casewhere the accumulation of absorption by the metal layer on the outermostsurface of the recess portion becomes a problem due to an extremelygreat scattering frequency and the case where the oscillationcharacteristic of the semiconductor laser itself is disadvantageouslychanged by return light due to coherent backscattering from the firstregion that includes scatterers at a high density.

A modification example the construction of the first region that avoidssuch problems and is effective in common to FIG. 4 of the secondembodiment and FIGS. 6A, 7A and 7C of the third embodiment will bedescribed next. This modification example can be preferably put intopractice even in the case where it is difficult to obtain a uniformdispersion of the scatterers of a comparatively small diameter. Forexample, it is proper to use the aforementioned scatterers as the main(maximum in terms of number density) and mix and disperse scatterers ofa relatively large diameter (q≧10) that deviates from the aforementionedrange (size parameter q is one to three) Also, in this case, thedispersion is achieved so that the average nearest neighbor distanceincluding all the scatterers falls within about twenty times the mode,i.e., the mode Ds of the scatterers mainly dispersed. With thisarrangement, there is obtained the effect of improving the outputefficiency from the second region while sufficiently reducing the amountof speckles. This is achieved by the synergistic effects of the factthat the asymmetry small scattering due to the main scatterers stilloccurs an extremely large number of times and the fact that the effectof wave front splitting and phase disordering of light can be expectedeven the subordinate scatterers exhibit a sharp forward scatteringcharacteristic. Typically, there was able to be found a dispersioncondition in which the problem of the hiding power was avoided bymaintaining the amount of speckles σ_(PAR) at 3×10⁻² even when themain-to-sub blend ratio was changed to at least about 9:1 to 8:2, andalmost no difference was recognized in the current-to-optical outputcharacteristic between a semiconductor laser in a chip state and theaforementioned various modules. As described above, there was found thecondition in which the dispersion process is facilitated by carrying outthe mixed dispersion in the first region, and eye safety was satisfiedwhile bringing the scattering characteristic of the first region closeto the characteristic supposed with regard to the main scatterers.

It is also acceptable to constitute the first region of a laminateconstructed of a plurality of two or more layers by arranging scatterersthat have different parameters spatially separately or spatiallychanging the dispersion density even in the case of the same scatterersin the first region instead of carrying out the mixed dispersion. Suchthe modification can be extremely easily carried out for the firstregions shown in FIG. 4 of the second embodiment and FIGS. 6A, 7A and 7Cof the third embodiment.

For example, in a situation that monodispersion of the scatterers havinga comparatively small particle size of a size parameter q of about 1 to15 at a density of 15 vol % is difficult and a sufficient scatteringcharacteristic cannot be obtained at a density of 15 vol % even ifuniform dispersion is possible, it is acceptable that ninety percent ofthe region adjacent to the semiconductor laser of the first region isconstructed of a 5-vol % monodispersion layer of the scatterers and athin layer in which the same scatterers are dispersed at a high densityof 50 vol % is provided in its uppermost ten percent of the region.

It was possible to obtain a sufficient speckle reducing effect whilerestraining the hiding power by principally producing the effect ofdividing the coherent wave front into a plurality of portions in theregion that occupies the greater part of the first region and thereaftergenerating sufficient multiple scattering in the uppermost layer. Bythus forming the first region into a multi-layer, the total cost becomesdisadvantageous although the difficulties in optimizing the dispersionprocess are reduced.

It is to be noted that the effect of the modification example of thefirst region described here is not limited to the solution of theproblem of the hiding power. For example, if the optical system can takea comparatively lose restriction on the size and a long total opticalpath length, it becomes possible to further facilitate the dispersionprocess or facilitate the handling by maintaining the viscosity afterthe dispersion process comparatively low.

In the structural examples described hereinabove, as is apparent fromthe configuration of the near-field pattern actually described, it isnot easy to obtain a constant average intensity distribution, i.e., anear-field pattern resembling a rectangle as an apparent light source.This is not always the necessary condition of eye safety. However, forexample, when the width of the near-field pattern of which the lightintensity becomes equal to or lower than 1/e is varied every lot due tothe fact that the configuration of the light intensity distribution isnot simple, the manufacturing yield is worsened, possibly causing a costincrease. Moreover, there is a tendency that the desired multiplescattering characteristic cannot be obtained when the thickness in theoptical axis direction of the multiple scattering optical system isextremely thin unless the dispersion is carried out at a density atwhich the hiding power becomes a problem. Moreover, when an extremelydistorted distribution configuration is possessed, it is sometimes thecase where the configuration and symmetricity of the radiant intensitydistribution (FFP) become impracticable.

Accordingly, an eye-safe light source device capable of solving theaforementioned problems and more effectively reducing speckles withhigher manufacturing yield in response to the demands for furtherreducing the size and thickness will be described next in the followingfourth embodiment.

FOURTH EMBODIMENT

FIG. 10A is a sectional view showing the construction of an eye-safelight source device as the light source device of the fourth embodimentof this invention. The eye-safe light source device of this fourthembodiment has a construction similar to that of the eye-safe lightsource device shown in FIG. 3A of the third embodiment except for theconfiguration of the recess portion.

In FIG. 10A, the principal wall surface portions of a recess portion1010 formed on a resin substrate 1003 are constituted so that thediameter thereof extends toward its opening and have a depth h=400 μmand a bottom surface diameter d=800 μm, defining an aspect ratio r(=h/d) of 0.5. Moreover, a wiring pattern 1012 is formed on the resinsubstrate 1003 and the outermost surface of the recess portion 1010 byAu plating. The lower surface of the semiconductor laser 1000 isdirectly fixed on the bottom surface (wiring pattern 1012) of the recessportion 101 with a silver paste. The wall surface is inclined so that anangle θ made between the normal line of the wall surface of the recessportion 1010 and the optical axis 1001 of the outgoing beam of thesemiconductor laser 1000 becomes θ=6° in terms of design, and assumingthat no scatterer is contained in the first region, the optical axis ofthe outgoing beam of the semiconductor laser 1000 is converted aplurality of times in the recess portion 1010 as shown in FIG. 10A.

It is proper to form the wall surface of such recess portion 1010through a process for penetrating, for example, a metal rod (not shown)that has the angle θ and a bottom surface diameter into the resinsubstrate 1003 while controlling the penetration depth. Subsequently,the wiring pattern 1012 is formed by carrying out underplating of nickeland copper and a main plating process of silver on the resin substrate1003 that has the recess portion 1010. Therefore, the metal layer, whichconstitutes the outermost surface of the recess portion 1010, is silver.Moreover, with this arrangement, the leakage of scattered light on thesubstrate 1003 side or in the directions other than the optical axisdirection 1006 of the second region can be restrained as described withreference to FIGS. 6A, 7A and 7C.

Subsequently, after an InGaAs/AlGaAs-based semiconductor laser 1000 on aGaAs substrate of an emission wavelength of 890 nm is wire bonded to thedie bond, acrylic crosslinked polymer particles of a particle size modeDs=4 μm are dispersed by 3 wt % in a thermosetting type silicone gel,and the resulting gel is injected into the recess portion 1010 andhardened to form a first region 1004 of a multiple scattering opticalsystem. Further, a second region 1005 of the multiple scattering opticalsystem is completed by transfer molding with epoxy resin.

FIG. 10B shows the relative light intensity distribution of thecomparatively satisfactory near-field pattern actually obtained by theeye-safe light source device of the aforementioned construction. Withregard to the construction of the principal wall surface of the recessportion 1010, according to:C _(min) ·r=1 andC _(max) ·r=20,the inclination angle θ [deg] satisfies:C _(min) ·r≦3≦θ≦C _(max) ·r.Even when the thickness in the optical axis direction 1006 of the secondregion 1005, i.e., the depth h has a small value of 400 μm, the specklescan be reduced by effectively generating the multiple scattering.Moreover, even by comparison with all the structural examples describedhereinbefore, it can be understood that the flatness of the intensitydistribution of the secondary planar light source formed at theinterface between the first region 1004 and the second region 1005 isremarkably improved. With this arrangement, the speckles can be reducedmost effectively within a minute volume.

Concurrently with this, a sharp configuration with no skirt trailing wasobtained at a radiation angle of a full width at half maximum of about30° with regard to the far-field pattern. Moreover, an extremely lowvalue of approximately 2×10⁻² was obtained as the amount of specklesσ_(PAR) of the near-field pattern, and a value of approximately 1×10⁻²was obtained as the amount of speckles σ_(PAR) of the far-field pattern.Furthermore, it was confirmed that the output efficiency was not reducedeven when the chip state of the semiconductor laser was compared withthe IL characteristic of the eye-safe light source device state by an IL(current-to-optical output) measurement system that was able to receiveonly the output light within a specified solid angle.

FIG. 11A shows the construction of another eye-safe light source devicethat reduces the speckles by effectively uniforming the near-fieldpattern similarly to the eye-safe light source device shown in FIG. 10A.In this case, the principal portion of the composite recess portion 1120in the first region is constructed of a cylinder 1110 of which thecross-sectional configurations of the opening portion and the bottomsurface portion are almost identical. A metal layer 1111 is formed onthe inner peripheral surface of the cylinder 1110 by Ag plating. Theentire recess portion 1120 is constructed including an Au plating wiringpattern 1112 and an Au bump 1113 on a resin substrate 1103, and itsoutermost surface is almost provided by a metal layer.

The cylinder 1110, which is the principal portion of the recess portion1120, has a height h=350 μm and an inside diameter d=800 μm, defining anaspect ratio r (=h/d) of 0.44. The cylinder 1110 was placed inclined onthe resin substrate 1103 so that an angle θ (inclination angle) madebetween the normal line of the metal layer 1111 and the optical axis1102 of the outgoing beam of the semiconductor laser 1100 becomesapproximately six degrees. In this case, becauser/5=0.09,tan θ=0.11, andr/2=0.22,the following relation:r/5≦tan θ≦r/2is satisfied. The inclination angle θ is relative, and the same effectcan be obtained also by making the axis of the principal portion(cylinder 1110) of the recess portion 1120 coincide with the normal lineof the resin substrate 1103 and die-bonding the semiconductor laser chipinclined instead. Anyway, it is easier to precedently die-bond thesemiconductor laser in terms of the order of mounting.

With regard to the wall surface of the recess portion 1120, themanufacturing processes and the devising for providing the inclinationcan be more easily carried out by rather subsequently attaching thecylinder 1110 prepared with the metal layer 1111 provided at least onthe entire inner wall surface onto the resin substrate 1103 that hasundergone a wiring patterning process, dissimilarly to the eye-safelight source device shown in FIG. 10A. In FIG. 11A, the inclinationangle θ is accurately secured by arranging the Au bump 1113 of a heightof about 80 μm on the Au plating wiring pattern 1112 and fixing the bumpwith a silver paste interposed between the pattern and the cylinder1110. Although it is easy to consider other various modificationexamples with regard to the provision of the inclination angle θ, it isrequired to form the entire recess portion 1120 without any gap betweenthe portion and the resin substrate 1103 in consideration of theinjection of a highly flowable material of gel, elastomer or the like inthe subsequent process. Moreover, with this arrangement, the leakage ofscattered light on the resin substrate 1103 side or in directions otherthan the optical axis direction 1106 of the second region can berestrained as described with reference to FIGS. 6A, 7A and 7C.

In the eye-safe light source device of this FIG. 11A, a first region1104 of a multiple scattering optical system is formed by injecting athermosetting type silicone gel in which styrenic crosslinked polymerparticles of a particle size mode Ds of 6 μm are dispersed by 6 wt %into the recess portion 1120. Further, a second region 1105 of themultiple scattering optical system is formed by transfer molding withepoxy resin.

FIG. 11B shows the near-field pattern actually obtained by the eye-safelight source device of the above-mentioned construction. In theconstruction of the principal wall surface of the recess portion 1120,the angle θ [deg] made between the normal line of the wall surface andthe optical axis of the outgoing beam of the semiconductor lasersatisfies the relation:max{a tan(r/5),3}≦θ≦a tan(r/2).Therefore, even when the height h of the cylinder 1110 (thickness in theoptical axis direction 1106 of the first region) had a short length of350 μm, the speckles were able to be reduced by effectively generatingthe multiple scattering. Similarly to the eye-safe light source deviceof FIG. 10A, it can be understood that the secondary planar light sourceuniformity is extremely high and the speckles of the near-field patternand the far-field pattern are extremely small (the amount of specklesσ_(PAR) of the near-field pattern is approximately 8×10⁻³, and theamount of speckles σ_(PAR) of the far-field pattern is approximately1×10⁻²). Moreover, it was confirmed that the output efficiency wasscarcely reduced when the chip state of the semiconductor laser 1100 wascompared with the current-to-optical output characteristic of theeye-safe light source device.

However, if a scatterer that singly had a large standardized scatteringcross-sectional area and a high isotropy was employed in the firstregions 1004 and 1104 in which multipath reflection and multiplescattering were used together as shown in FIG. 10A or 11A of this fourthembodiment, there were the tendencies that diffusion in the directionperpendicular to the optical axes 1006 and 1106 of the second regions1005 and 1105 did not sufficiently occur, and it was difficult to obtaina uniform near-field pattern intensity distribution as a secondaryplanar light source, when the speckles were not sufficiently reduced.Moreover, there was found the tendency that the output efficiency wasreduced even a uniform near-field pattern could be obtained, and theinfluence of the hiding power was actualized. Conversely, it wasdiscovered that excessive speckles (e.g., σ_(PAR)>3×10⁻¹) were possiblygenerated even though the dimensions of the portions and the type,density and mixture ratio of the scatterers were optimized when theinclination angle θ exceeded about 10° in the construction of FIG. 10Aor when the inclination angle θ exceeded about 15° in the constructionof FIG. 11A according to the evaluation results of the modules in whichpreferable scatterers of a comparatively large particle size of a sizeparameter q of about 10 to 50 were employed and the inclination angle θwas changed.

Furthermore, when a scatterer of a comparatively large particle size ofa size parameter q of about 10 to 50 was employed and the inclinationangle θ was set to 0°, there were many eye-safe light source devices inwhich a radiant intensity at a wide angle occurred, and the opticaloutput was reduced by 10% or more through the measurement of totalquantity of light by means of an integrating sphere although the fullwidth at half maximum of the far-field pattern was not significantlychanged in each of the constructions of FIGS. 10A and 11A. This ismainly ascribed to the fact that the cumulative effect of minuteabsorption of scattered light incident on the wall surface of the recessportions 1010 and 1120 at an approximately right angle. It was found tobe desirable to provide an inclination angle θ of not smaller than atleast 3° in order to avoid the problem that the radiant intensityoccurred at the wide angle although the inclination angle θ could take avery small value depending on the aspect ratio r of the recess portions1010 and 1120 and the problem that the optical output was reduced by 10%or more through the measurement of total quantity of light by means ofthe integrating sphere.

As described above, in the case of the first region of FIG. 10A in whichthe multipath reflection and the multiple scattering are used, becauseC _(min) ·r=1 andC _(max) ·r=10,the principal wall surface of the recess portion 1010 is constituted atan aspect ratio r such that the inclination angle θ satisfies:C _(min) ·r≦3≦θ≦C _(max) ·r.Moreover, in the case of FIG. 11A, the principal wall surface of therecess portion 1120 is constituted at an aspect ratio r such that theinclination angle θ satisfies:max{a tan(r/5),3}≦θ≦a tan(r/2).Further, it was found that the aforementioned plurality of conflictingrequirements were able to be satisfied when the scatterers dispersedmainly in the first region were made to have a comparatively largeparticle size of a size parameter q of about 10 to 50.

Moreover, it is possibly the case where the speckle reducing effectcannot sufficiently be obtained although the light source size is easilyenlarged and flattened by employing the scatterers of a comparativelylarge particle size as described above. In such a case, it is proper todisperse scatterers of a comparatively low refractive index differencelike, for example, acrylic particles at a density of mainly 1 vol % to30 vol %, typically about 10 wt %, further add subordinate particles(e.g., TiO₂ particles having an average diameter of 0.3 μm) having highscattering abilities at a ratio of 0.1 vol %, the subordinate particleshaving a refractive index difference Δn≧0.2 and a size parameter q≦10,and mix and disperse those particles. A condition for solving theaforementioned problem that the speckle reducing effect could notsufficient be obtained was able to be found by setting the volume ratioof the subordinate scatterers to be mixed and dispersed to approximately0.01 vol % to 0.1 vol % and setting the range up to 1 vol % according tocircumstances.

Various scatterers are applied to the constructions of FIGS. 10A and11A, and the results concerning the near-field patterns that haveremarkable differences from particularly the aforementioned otherconstructions are shown in FIG. 12. In particular, the resultsconcerning the mixed dispersion of the main scatterers of a lowrefractive index difference and the subordinate scatterers of a highrefractive index difference are additionally shown. In this case, thehorizontal axis of FIG. 12 represents the transport optical depthL/l_(AVE) similar to that of FIG. 5A or 8, and the plotting of thegeometrical dimension L is changed (see FIGS. 10A and 11A). This isbecause the definition of the length according to the figures is almostmeaningless in the actual multiple scattering optical system althoughFIGS. 10A and 11A schematically show the state in which the optical axisof the outgoing beam of the semiconductor laser is converted assumingthat there is no scatterer. Therefore, evaluations were made by settingthe thickness of the first regions 1004 and 1104 and the second regions1005 and 1105 to L in the direction of the optical axes 1006 and 1106.

As the remarkable feature of FIG. 12, it can be enumerated the casewhere SiO₂ and acrylic crosslinked particles of a low refractive indexdifference Δn might be effective for further reducing the amount ofspeckles σ_(PAR). Typically, a sufficient reduction in the amount ofspeckles σPAR was achieved within a density range of about 5 vol % to 30vol %. Moreover, according to the constructions of FIGS. 10A and 11A,the re-increase in the amount of speckles in the case of high densitydispersion is not remarkably observed, and a tendency of saturation isexhibited. Such the effect is caused by the flattening of the near-fieldpattern and the easiness of dispersion uniformity due to the use of theparticle that has a comparatively low refractive index difference and alarge particle size.

Even in the case of TiO₂ and the styrenic particles having acomparatively large refractive index difference, it is possible tosecure eye safety by increasing the particle size and applying the sameto the construction of this eye-safe light source device. However, inthe case of particularly TiO₂ particles, the influence of the hidingpower was particularly large, and the producible optical output wasextremely reduced in a dispersion density range in which the speckleswere further reduced according to the results of FIGS. 7B and 7D.Moreover, in contrast to the fact that the amount of speckles σ_(PAR)tended to saturate at a density of about 10⁻² in the case of thedispersion of the SiO₂ particles (symbol x in FIG. 12) only or theacrylic particles (symbol Δ in FIG. 12) only, a more excellentcharacteristic was obtained by dispersing the subordinate scatterers ofa comparatively small particle size and a high refractive index inmixture at a density of about 0.01 vol % to 0.1 vol % (+: SiO₂+TiO₂, ▴:acrylic+TiO₂), and little influence was received from the hiding power.

As described above, it was found to be preferable to mix and dispersethe main scatterers of a comparatively low refractive index difference(typically about 0.05≦Δn≦0.2) and a large particle size (q≧approx. 10)with the subordinate scatterers of a high refractive index difference(Δn≧approx. 0.2) and a small diameter (q≦approx. 10) like theaforementioned TiO₂ in the construction that used a combination of themultipath reflection of the wall surface of the recess portion and themultiple scattering of the scatterers. By blending at a comparativelylow density within the permissible range from the viewpoint of specklereduction, a sufficient speckle reducing effect can be obtained bymaking the operation of uniforming the intensity distribution of thesecondary planar light source of the main scatterers compatible with theoutput efficiency through a simpler dispersion process.

Moreover, it is acceptable to dispersedly arrange different scatterersspatially separated as described above or to constitute the first regionof a laminate constructed of two layers or a plurality of layers bychanging the dispersion densities of the layers instead of carrying outthe aforementioned mixed dispersion. For example, it is acceptable toconstitute 90 percent of the region located adjacent to thesemiconductor laser out of the first region of a dispersion layer thathas the scatterers at a density of 10 vol % and provide a thin layer inwhich scatterers that have a comparatively small particle size and ahigh refractive index difference of a size parameter q of about 1 to 10are dispersed at a density of 1 vol % in the 10 percent of the region inits upper portion when a sufficient multiple scattering characteristiccannot be obtained even if scatterers of a comparatively large particlesize of a size parameter q of 10 to 50 are uniformly dispersed at adensity of 15 vol %. It is sometimes the case where the preferablemultiple scattering characteristic can be found by optimizing theaforementioned conditions (size parameter q and dispersion density) evenif the uppermost layer of the multi-layered first region is apolydispersion including agglomerates. By thus forming the first regioninto a multi-layer, the total cost becomes disadvantageous although thedifficulties in optimizing the dispersion process are reduced.

Although the semiconductor lasers that emits light from one end surfaceare employed for the sake of simplicity in FIGS. 10A and 11A, theuniformity of the near-field pattern can be improved more easily byemploying a semiconductor laser of the type that emits light from bothend surfaces in each of the constructions. With regard to the specklereducing effect, there is observed no significant difference between onepoint light source and the two point light sources although thesemiconductor laser of the type that emits light from both end surfaceshas a slight superiority. Therefore, it is preferable to employ thehigh-power semiconductor laser, which emits light from one end surfaceand of which the mass production technology has been established foroptical disc applications and so on in terms of cost. Moreover, as amodification example of the wall surface of the recess portion thatforms the first region, the cross-sectional configuration of thecylindrical portion viewed from above in the optical axis direction ofthe lens portion of the second region is not always required to becircular. It is desirable that the cross-sectional configuration of thecylindrical portion is axially symmetrical to the optical axis of thesemiconductor laser. However, the configuration may be an arbitraryconfiguration such as a polygon, and the spatial coherency can bereduced more effectively by dividing the optical axis of thesemiconductor laser into at least three or a plurality of portions withthis arrangement.

Although the first region and the second region, which are theconstituent elements of the multiple scattering optical system extremelyeffective for reducing the coherency of the high-power semiconductorlaser have been described in detail hereinabove, the characteristics ofthe semiconductor laser itself of another constituent element has notbeen described in detail. The characteristics of the semiconductor laseremployed in this eye-safe light source device will be described below.

As already described above, the typical spectral linewidth of the singlebody of a semiconductor laser is about 10 MHz, and the coherence lengthis at least several meters. The detailed description of the multiplescattering optical system described hereinabove is actually based on theevaluation results of the eye-safe light source device equipped with thesemiconductor laser of such characteristics.

Particularly, as a high-power semiconductor laser as a communicationlight source suitable for an Si detector, the GaAs/AlGaAs-based ridgestripe structure is most common, and the semiconductor laser of the780-nm band conventionally mass-produced for CD-R/RW has sometimes beenused without modification because of an advantage in terms of cost. As asemiconductor laser to be mounted on a light source device for wirelessoptical communications of high-speed and high-power operation supposedby this invention, the existing 780-nm band lasers for optical discs andthe 980-nm band lasers for EDFA (Erbium-Doped Optical Fiber Amplifier)are not always be optimum.

In the wavelength band of 880 nm to 920 nm, the threshold current andthe temperature characteristic of the semiconductor laser are remarkablyimproved in comparison with those of the 780-nm band device in additionto the fact that the wavelength band is close to the peak sensitivitywavelength of Si. Typically, in contrast to the fact that the thresholdcurrent of the 780-nm band device of a cavity length of 600 μm is about35 mA, a current of 15 mA and a temperature characteristic value T0 ofnot smaller than about 160 K are obtainable at the wavelength of, forexample, 890 nm. Such characteristics, which can be theoreticallyexpected from the characteristics of the InGaAs active layer element ofthe 980-nm band, have been actually scarcely examined in detail for asemiconductor laser. The present inventors conducted examinations forthe device structure paying attention to the coherency of thesemiconductor laser while attempting a cost reduction by establishing amaximum compatibility with the 780-nm AlGaAs-based semiconductor laserin terms of the manufacturing processes. As a result, it was found thatthe following prominent operation and effects were able to be obtainedby adopting a GaAsP ternary system material or an InGaAsP quaternarysystem material advantageous to higher output operation with an Al-freearrangement for a quantum well barrier layer or a light guide layer inaddition to the use of the active layer including InGaAs.

For example, a light guide layer is provided in the layer structure of asemiconductor laser, and the material of the light guide layer isprovided by InGaAsP. In the InGaAsP mixed crystal system serving as thelight guide layer on the GaAs substrate, a miscibility gap (growthcondition incapable of obtaining a complete mixed crystal) exists.Spatial fluctuations can be generated in the composition of the lightguide layer taking advantage of this phenomenon. Timewise fluctuationsare generated in the laser emission wavelength by this action, andtimewise coherency is reduced. It was found that speckles were able tobe reduced extremely effectively by combining such a semiconductor laserwith the multiple scattering optical system of this invention.

Typically, if crystal growth is carried out by using the reducedpressure MOCVD (Metal Organic Chemical Vapor Deposition) method, and thephotoluminescence of the single body of light guide layer is subjectedto mapping in the wafer, it is possible to find a crystal growthcondition in which the deviation of the peak wavelength intraplanardistribution becomes about 5 nm to 30 nm and the average size of itsspatial distribution becomes about 10 μm. For example, it is proper toreduce a growth temperature Tg by about 50° C. to 200° C. with respectto the normal growth optimum temperature for obtaining a uniform qualitythin film. As a result, it was confirmed that the compositionalfluctuations were able to be generated without causing a reduction inthe crystalline quality to such an extent that optical loss was causedin the light guide layer according to the device characteristics.

In this case, the normal growth optimum temperature (condition) is acondition in which the intraplanar uniformity of photoluminescence andits intensity almost become optimum, and the temperature is generallyset higher than the spinodal decomposition temperature by 100° C. to150° C. Moreover, the temperature is not set higher than the range of100° C. to 150° C. so as to achieve matching with the growth conditionsof other layers and particularly the active layer.

The condition in which the compositional fluctuations are generated isbasically found through crystal growth at the spinodal decompositiontemperature or a temperature below it. However, there are manyparameters such as a flow channel configuration that greatly depend oneach individual crystal growth system, and it is difficult to quantify auniversal condition including a gas flow rate or the like.

FIGS. 13A and 13B are sectional views showing the concrete structure ofa semiconductor laser that has layers as described above. First of all,FIG. 13A will be described in detail.

As shown in FIG. 13A, an undoped DQW (Double Quantum Well) active layeris constructed of a 80-Å thick In_(0.074)Ga_(0.926)As well layer 1301, a50-Å thick In_(0.100)Ga_(0.900)As_(0.657)P_(0.344) barrier layer 1302(tensile strain: −0.5%) and a 200-Å thick light guide layer 1303(tensile strain: −0.5%) having the same composition as that of thebarrier layer 1302. In the semiconductor laser of the above-mentionedstructure, the InGaAsP light guide layer 1303 mainly provides the actionof compositional fluctuations according to the relation of the opticalconfinement coefficient. Such the active region is wholly grown at acomparatively low temperature of 515° C. The crystal mixture ratio ofthe light guide layer 1302 is an average value obtained from EPMA (X-raymicroanalysis) when the normal crystal growth at a temperature higherthan the spinodal temperature at which the compositional fluctuations donot occur is carried out. The energy gap of the barrier layer 1302corresponds to Al_(0.23)Ga_(0.77)As. Moreover, the crystal growthtemperature is shifted to the optimum growth temperature of 770° C. ofan ordinary AlGaAs-based material in the 30-Å thick undopedAl_(0.25)Ga_(0.75)As second light guide layer 1304 provided on bothsides of the undoped DQW active layer.

The following describes the details of the layers optimized to theemission wavelength of 890 nm as an example of the whole devicestructure. In FIG. 13A, there are shown a 0.1-μm thick p-typeAl_(0.40)Ga_(0.60)As third light guide layer 1305, a 0.1-μm thick n-typeAl_(0.40)Ga_(0.60)As third light guide layer 1306, a 0.135-μm thickp-type Al_(0.50)Ga_(0.50)As first cladding layer 1307, a 0.20-m thickn-type Al_(0.50)Ga_(0.50)As first cladding layer 1308, a 30-Å thickundoped GaAs etching stop layer 1309, a 1.28-μm thick p-typeAl_(0.478)Ga_(0.522)As second cladding layer 1310, a 2.3-μm thick n-typeAl_(0.425)Ga_(0.575)As second cladding layer 1311, an n-typeAl_(0.70)Ga_(0.30)As block layer 1312, a p++ type GaAs cap layer 1313and an n-type GaAs layer 1314 as a buffer and a substrate.

The semiconductor laser 1300 was applied to the multiple scatteringoptical system of which the construction was shown in FIG. 6A of thethird embodiment. Styrenic crosslinked particles of an average particlesize of 0.8 μm similar to those of FIG. 6A are used as the scatterers,which are dispersed at a density of 10 wt % in a thermosetting typesilicone gel used as the dispersion base material. The amount ofspeckles σ_(PAR) of the eye-safe light source device that employs thissemiconductor laser 1300 was reduced to approximately ½ to 1/10 (notshown) in comparison with the value obtained with the construction ofthe eye-safe light source device of FIG. 6A with regard to both thenear-field pattern and the far-field pattern. The semiconductor laserspreviously shown as the constituent elements of FIGS. 6A, 7A, 7C, 10Aand 11A have a structure constructed of only the classic GaAs/AlGaAs forCD-R/RW except for the fact that active layer well layer is the sameInGaAs DQW. That is, the device is a laser device in which thecompositional fluctuations as in the semiconductor laser 1300 do notexist.

As described above, it was clarified that the semiconductor laser 1300having the spatial compositional fluctuations of the InGaAsP layer wasextremely effective for speckle reduction as the constituent element ofthe multiple scattering optical system of this invention. The eye-safelight source device equipped with the semiconductor laser 1300 exhibiteda spectral band width of about 1.5 nm as a timewise integral valuealthough it was single-humped during the CW operation at 100 mW, and adevice lifetime of not shorter than 2000 hours was obtained at least at70° C. with this characteristic maintained. Moreover, it was also foundthat the expansion of the spectral linewidth became more remarkable ifthe injection current was modulated with a large amplitude of 100mA_(p-p).

Then, the eye-safe light source device of FIG. 6A of the thirdembodiment, the semiconductor laser 1300 and the eye-safe light sourcedevice of FIG. 6A equipped with the semiconductor laser 1300 were eachsubjected to the measurement of the coherence length Lc by means of aMichelson interferometer through a spatial filter. As a result, it wasconfirmed that the coherence length Lc was reduced by one or more ordersof magnitude in the two devices that employed the semiconductor laser1300 as a light source element, and the coherence length Lc was shorterin the form of a module between these two devices. That is, it was alsoconfirmed that return light was generated on the optical axis of thesemiconductor laser due to the coherent backscattering peak since thefirst region of the multiple scattering optical system of this inventionis located adjacent to the semiconductor laser in addition to the factthat the timewise coherency was significantly reduced in comparison withthe normal laser in the semiconductor laser 1300, and this had theeffect of reducing the timewise coherency independently of the operationof increasing the spectral linewidth of the semiconductor laser 1300itself.

As described above, the timewise coherency can be reduced by introducingthe light guide layer that has spatially minute compositionalfluctuations inside the semiconductor laser and giving dynamicfluctuations to the oscillation frequency (wavelength).

Further, the structure of another semiconductor laser that has a similaroperation will be described with reference to FIG. 13B. The samereference numerals are given to the same layers as those of FIG. 13A. Inthis case, a p-type In_(0.379)Ga_(0.621)As_(0.251)P_(0.749) light guidelayer 1323 is newly provided apart from the active region, and thecompositional fluctuations are generated only in the light guide layer1323. In order to secure the optical confinement coefficient andremarkably obtain the effect of reducing the timewise coherency, thelight guide layer 1323 is made to have a comparatively great thicknessof 450 Å and roughly lattice matched to the GaAs substrate. The growthtemperature of the light guide layer 1323 was lowered to 680° C.(spinodal decomposition temperature is about 780° C.). Moreover,changeover of the growth temperature between the above-mentioned layerand the (Al)GaAs layers 1307 and 1309 located on both sides is achievedby providing a waiting time in the crystal growth stage of the (Al)GaAslayers 1307 and 1309. Moreover, the barrier layer 1321 and the firstlight guide layer 1322 in the active region in this case are made ofundoped Al_(0.25)Ga_(0.75)As and grown at the same growth temperature of650° C. as that of the InGaAs layer 1301.

It was clarified that an almost similar effect of reducing the timewisecoherency was obtained by approximately equating the optical confinementcoefficient of the light guide layer 1303 and the light guide layer 1323that had equivalent compositional fluctuations as a result of performingspeckle evaluation similar to that of FIG. 13A by means of thesemiconductor laser 1320 that had the device structure of FIG. 13B.

The introduction of the InGaAsP layer that has the compositionalfluctuations in place of the normal AlGaAs layer in the structure of thesemiconductor laser is not limited only to the layer shown in the twosemiconductor lasers of FIGS. 13A and 13B. That is, the above-mentionedarrangement can be applied to any one or an arbitrary combination of thequantum well barrier layer 1302, the light guide (SCH) layer 1303located adjacent to it and the light guide layer 1322 newly provided.Particularly, for the comparatively thin barrier layer 1302 inside themultiquantum well structure or the like, there can be employed a ternarysystem represented by Ga_(1-X)As_(Y)P_(1-Y) (0<X<1, 0<Y<1) including thelattice mismatch system or a quaternary system represented byIn_(X)Ga_(1-X)As_(Y)P_(1-Y) (0<X<1, 0<Y<1). Otherwise, in the laserdevice that attaches importance to the temperature characteristic ratherthan a high power, it is effective to add, for example, an AlGaAsSblayer that has a distortion as a layer for blocking the overflow ofelectrons from the active layer to the inside of the active layer or itsneighborhood. Also, in this case, it is needless to say that the effectsof the InGaAsP layer on speckle reduction can be obtained quitesimilarly independently of an improvement in the temperaturecharacteristic. Furthermore, it is possible to generate spatialfluctuations in the layer thickness and the composition also in theaforementioned AlGaAsSb electron blocking layer by devising variouscrystal growth conditions of MOCVD, MBE (Molecular Beam Epitaxy) or thelike.

Further, by intentionally carrying out three-dimensional growth in anisland-like configuration and remarkably generating the intraplanardistribution of layer thickness during the crystal growth of thestrained quantum well layer 1301 (InGaAs), a pseudo gain grating can beformed. As a result of performing evaluation in a module state similarto the above, there was observed a speckle reducing effect due to thespectral linewidth increase operation similarly to the aforementionedInGaAsP layer. However, there was observed the tendency that thelinearity of the IL characteristic was impaired when the pseudo gaingrating was employed.

It is needless to say that a further coherency reducing effect can begenerated by combining layer structures that have spatial fluctuationsin at least one of the composition or layer thickness inside thesemiconductor laser. However, from the viewpoint of the reliability ofthe semiconductor laser, it is rather preferable to obtain the spectrallinewidth increasing effect by the InGaAsP light guide layer 1323 or thelike located apart from the active region and carry out growth forobtaining the satisfactory crystalline quality of the quantum well layer1301 itself.

The expansion of the spectral linewidth due to the pseudo gratingoperation as described above is attributed to the fact that the phase ofthe structure in the real space is obscure as the whole cavity or thefact that the particle size is typically a few times as large as thewavelength and the Bragg condition is not accurately satisfied as isapparent from its formation method. On the other hand, in comparisonwith the semiconductor laser constituted of only a layer that hascomplete smoothness or no (or extremely little) compositionalfluctuations, the quantity of overlap of the spatial fluctuations of thegain or equivalent refractive index with the stationary wave severelychanges every longitudinal mode. Therefore, the threshold gain becomesminimized in a specified mode, and the tendency of single longitudinalmode oscillation occurs. In fact, according to the measurement exampleof the single body of the laser chip, the laser chip operated with thesingle longitudinal and transverse modes maintained up to at least CW100 mW, and the linearity of the current-to-optical outputcharacteristic was also satisfactory. Although kink or mode hoppingsometimes occurs at a high power attributed to the aforementioned effectdepending on the device, there is obtained a sufficient characteristicfor carrying out intensity modulation with a large amplitude in a binaryform like wireless optical communications.

FIFTH EMBODIMENT

This invention is not limited to the molded products that have beenconcretely described hereinabove and lead frame base products on theresin substrate in using an arbitrary semiconductor laser in combinationwith the multiple scattering optical system unlimitedly to thesemiconductor lasers 1300 and 1320 of which the spectral linewidth areextended as shown in FIGS. 13A and 13B. That is, as shown in FIG. 14,the multiple scattering optical system of this invention can beconstituted by employing a small-sized CAN package of a diameter of 3.5mm and 5.6 mm.

As shown in FIG. 14, a semiconductor laser 1400 is die-bonded andwire-bonded to a stem 1403 similarly to an ordinary packaging. Further,a first region 1404 of the multiple scattering optical system is formedby internally filling a cap 1410 with a silicone gel in which scatterersare dispersed. Particularly, when the diffused light scatters to theinner wall surface of the cap 1410, it is preferable to avoid theproblem of accumulation of photoabsorption by the inner wall portionpeculiar to the multiple scattering optical system. That is, it isproper to provide the inner wall of the cap 1410 by a metal layer 1411that has a high reflectance to incident light at all angles bysubjecting the wall to surface processing of Au plating or silver pastecoating instead of the normal Ni plating or the like. With thisarrangement, a further speckle reducing effect and near-field patternflatness are remarkably improved as already described.

Moreover, an ordinary cap sealing process is changed to theaforementioned silicone gel hardening process in this case. Further, anepoxy resin layer 1405 having lens operation is formed as the magnifierof the second region of the multiple scattering system on a cap glass1406. As described with reference to FIGS. 9B and 9C, by adjusting thedie-bonding position of the semiconductor laser on the stem, thethickness and the radiuses of curvature of the epoxy layer so that thedesired radiant intensity distribution can be obtained, a distance fromthe secondary planar light source formed at the interface between thefirst region 1404 and the cap glass 1406 to the top of the epoxy resinlayer 1405 that is the lens portion can be set to a preferable value.

The difference of the semiconductor laser structure of theaforementioned multiple scattering optical system was evaluatedsimilarly to the evaluation performed for the mold module as describedabove. As a result, it was confirmed that the amount of speckles σ_(PAR)of the near-field pattern was reduced to the level of at least about ⅓to 1/10 when the semiconductor laser 1300 (shown in FIG. 13A) wasmounted in comparison with the case of an ordinary InGaAs/AlGaAs-basedlaser free from compositional fluctuations or the like.

In this case, in the construction of the multiple scattering opticalsystem described in FIGS. 2 through 12 of the first through fourthembodiments, the first region and the second region have been brought indirect contact with each other, and the secondary planar light sourcewith effectively reduced coherency has been formed at the interface. Incontrast to this, in the eye-safe light source device of FIG. 14, thisfirst region 1404 and the epoxy resin layer 1405 are put in contact witheach other with interposition of the cap glass 1406. However, as isapparent from the purport of this invention that has been describedhereinabove, the region of the combination of the epoxy resin layer 1405and the cap glass 1406, which contain no scatterer, operate as thesecond region of the multiple scattering optical system in this case.

More in detail, the scattered light emitted from the secondary planarlight source formed at the interface between the first region 1404 andthe cap glass 1406 is incident on the glass layer (1406) of a refractiveindex of 1.6 from the region in which the silicone gel of a refractiveindex of 1.4 serves as the base material and further incident on theepoxy resin layer 1405 of a refractive index of 1.5. Although adiscontinuous refractive index plane exists at the interface between theglass layer (1406) and the epoxy resin layer 1405, a paraxial focalpoint is formed of these two layers (cap glass 1406 and epoxy resinlayer 1405) on the semiconductor laser 1400 side similarly to thepreviously described constructions, and the fact that the position islocated deeper than the secondary planar light source can easily beshown. Therefore, it is evident that the two layers of the glass layer(1406) and the epoxy resin layer 1405 totally operate as the secondregion of the multiple scattering optical system of this invention.

As described above, the first region and the second region of themultiple scattering optical system of this invention can be subjected tothe modifications of forming the inside of the first and second regionsinto a multi-layer and so on so long as the regions have the operationsdescribed hereinabove. It is to be noted that, when a layer having arefractive index relatively lower than that of another region inside thesecond region or the base material of the first region and a greatrefractive index difference of, for example, not smaller than 0.1 isincluded in the second region, the scattered light incident from thefirst region is totally reflected at a considerable rate, and theprocess of re-incident on the first region from the second regionbecomes remarkable. In this case, the effect of reinforcing thescattering characteristic of the first region is obtained, and theinfluence of the hiding power is actualized according to circumstances.However, by optimizing the portions in correspondence with this, apreferable multiple scattering optical system can be obtained.

SIXTH EMBODIMENT

FIG. 15A is a sectional view showing the construction of an opticalcommunication module for wireless optical communications employing theeye-safe light source device of the sixth embodiment of this invention.

First of all, in an embodiment as shown in FIG. 15A, a semiconductorlaser 1502 of an emission wavelength of 890 nm, a drive IC (IntegratedCircuit) 1506 in which a drive circuit and a reception circuit areintegrated and a 250-μm thick Si-pin photodiode 1507 are mounted asactive elements on a 500-μm thick FR4 substrate (glass epoxy resinsubstrate) 1501 that serves as a support of the optical communicationmodule. The semiconductor laser 1502 is provided with the structuralfeatures of an average stripe width of 2.5 μm, a cavity length of 500μm, an AR/HR end surface coating, an InGaAs single quantum well activelayer and so on and has a COD level of 280 mW.

The semiconductor laser 1502 is die-bonded to a metal base 1503 on whicha recess portion 1503 a that has a conical mounting top and a wallsurface of an inclination angle of 45° similarly to the aforementioneddescription is formed. Moreover, the metal base 1503 has a totalthickness of 500 μm, the recess portion 1503 a has a depth of 300 μm,and smooth Ag plating is provided on its entire outermost surface.Although no description is provided for the details of the overallcircuit configuration of the optical communication module, portions areelectrically connected via an Au wiring pattern on the FR4 substrate1501 by wire bonding. Moreover, terminals of Vcc, GND, Tx/Rx and so onare drawn out and arranged for soldering from a side surface to the backsurface of the FR4 substrate 1501.

Further, the recess portion 1503 a of the metal base 1503 is filled witha silicone gel in which TiO₂ of an average particle size mode Ds of 0.3μm (q=2.1) is dispersed by a percentage by weight of 2 wt %, forming afirst region 1504 of the multiple scattering optical system.Particularly, by injecting a dispersion gel (silicone gel) so as tocover also the upper surface of the opening of the recess portion 1503a, the Au wire 1510 and the metal base 1503 are effectively preventedfrom being short-circuited. Moreover, the entire optical communicationmodule is sealed by transfer-molding an epoxy resin that has notundergone intentional scatterer dispersion. A sufficient number ofsamples were evaluated, and there were obtained a full width at halfmaximum of about 1.2 mm and a speckle amount average value<σ_(PAR)>=0.025±0.001 as average values including measurement errorswith regard to the near-field pattern. Moreover, the far-field patternwas a sharp cutoff pattern free from a skirt trailing, and there wereobtained a full width at half maximum of about 300 and a speckle amountaverage value <σ_(PAR)>=0.022±0.001.

In this case, the particularly important feature in the embodiment ofFIG. 15A is that the first region 1504 is located adjacent to thesemiconductor laser 1502 and exists only inside the recess portion 1503a of the metal base 1503. With this arrangement, the second region 1505of the multiple scattering optical system that is the transmissionsystem lens portion and the aforementioned sealing resin including areception system lens 1508 can be collectively formed through anidentical process in addition to the spatial (or timewise) coherencyreducing effect in the first region 1504 as described hereinabove. Thatis, no scatterer is contained in the sealing resin including thetransmission system lens portion (1505) and the reception system lens1508, and therefore, the configuration of the reception system lens 1508and a distance to the photodiode 1507 can be optimally designedindependently of the multiple scattering optical system on thetransmission side.

As described above, it is possible to concurrently provide atransmission section that has a multiple scattering optical system andsatisfies the Class 1 eye safety and a reception section that includesno multiple scattering optical system and makes a wider FOV (Field ofView) and a high sensitivity compatible. Moreover, it was confirmed thatthe optical communication module 1500 of FIG. 15A was manufactured withhigh yield as a subminiature electric component having a total thicknessof 1.6 mm including the FR4 substrate and widths of 7 mm×2 mm andsufficient reliability was possessed under the use condition of −40° C.to +85° C.

FIG. 15B shows a current-to-optical output characteristic at the roomtemperature in the transmission section of the optical communicationmodule 1500. An operating current was 120 mA and an operating voltagewas 1.8 V during the typical 100-mW operation (on-axis radiantintensity: 295 mW/sr). Moreover, the bandwidth of the transmissionsystem was measured directly from a terminal not via the driver IC 1506,and it was confirmed that there was no power penalty up to at least 500MHz that was the limit of the measurement device, and the speed-uppotential was not impaired at all as a light source equipped with asemiconductor laser.

In connection with the first through sixth embodiments, the eye-safelight source devices each of which employs a single narrow-stripehigh-power semiconductor laser have been described as a semiconductorlight-emitting device. However, the semiconductor light-emitting devicethat has a superiority in the photoelectric conversion efficiency withrespect to LED, is not limited to this, and it is acceptable to applythis invention to light source devices that employ various light sourceelements of varied timewise and spatial coherencies, such as an arraylaser of a plurality of stripes, a broad area laser, SLD and so on.

According to the light source device of this invention, by adopting asemiconductor laser of the near infrared region of which the massproduction effect is high and the output increase technology is advancedin a small-sized resin-sealed optical communication module equipped witha semiconductor laser, there can be concurrently achieved low-currenthigh-power operation, high-speed operation and the securing of specklefree Class 1 eye safety at a minimum cost. As a result, the key deviceof an inexpensive, high-speed wireless optical communication system canbe provided without sacrificing convenience by controlling the beamemission angle.

Particularly, by adopting a semiconductor laser that has InGaAs providedin the active layer and oscillates within the wavelength range of 880 nmto 920 nm as a semiconductor light-emitting device of the light sourcedevice of this invention, the threshold current and the operatingcurrent are extremely lowered, and high-power operation is stablycarried out with an APC (Automatic Power Control) free or bias freesimple circuit. Further, by employing an inexpensive Si photodiode forthe photodetector, the cost performance of the wireless opticalcommunication system can be remarkably improved.

The effects obtained by the individual constructions of the multiplescattering optical systems of the light source devices disclosed in thisinvention are as follows.

The multiple scattering optical system constructed of the first regionthat contains the scatterers at a relatively high density and is locatedadjacent to the semiconductor laser and the second region that islocated adjacent to the first region and extended to a free space isprovided, and the second region is made to operate as a magnifier withrespect to at least the principal portion of the secondary planar lightsource formed at the interface between the first region and the secondregion. With this arrangement, by effectively generating multiplescattering that reduces spatial coherency of the laser beam in theextremely minute first region and mainly controlling the angulardistribution characteristic of the radiant intensity in the secondregion, the optimization of the portions can be separately achieved.Moreover, by setting the density of the scatterers included in thesecond region to one tenth or less of that of the first region, theunnecessary skirt trailing outwardly of the half-value angle of theradiant intensity is restrained. Thus, the eye safety of the near-fieldpattern and the shaping of the far-field pattern can be made compatiblewith a simple optical system.

Moreover, by undergoing multiple scattering at least a few times as atransport optical depth in the minute first region located adjacent tothe semiconductor laser, the single secondary planar light source thathas the desired apparent light source size at the interface between thefirst region and the second region is formed, and the Class 1 eye safetycan be secured. That is, the local peak structure of the near-fieldpattern that has an expansion of about 0.01 mm to 0.1 mm is scaled downin size and made indistinct. Moreover, the probability distribution ofthe PAR amplitude of the near-field pattern comes to be regarded as aGaussian distribution, and the amount of speckles σ_(PAR) can be reducedto an extremely low level on the order of 10⁻² or below it.

Moreover, the second region operates as the magnifier for at least theprincipal portion of the secondary planar light source formed at theinterface between the first region and the second region, theprobability of the occurrence of local overlap of optical paths at a lowangle is reduced through the process of conversion from the near-fieldpattern to the far-field pattern from the second region to the freespace while efficiently collecting also the scatter components at acomparatively wide angle. Therefore, a far-field pattern of a highuniformity of radiant intensity and sharp cutoff is formed with thefar-field pattern speckles restrained. As described above, it ispossible to make a preferable optical characteristic compatible with thesecuring of eye safety even in the minute multiple scattering opticalsystem.

Moreover, the diameter mode Ds of the scatterer that is the constituentelement of the first region has its size parameter q selected from therange of approximately 1 to 50. In the multiple scattering opticalsystem that uses no multipath reflection, the size parameter q is ascatterer that satisfies the range of 1 to 15 and more preferably therange of 1 to 10, and scatterers roughly having a refractive indexdifference Δn≧0.15 with respect to the dispersive medium are mainlydispersed. Moreover, in the multiple scattering optical system thatconcurrently uses multiple scattering and multipath reflection,scatterers having a comparatively low refractive index difference of asize parameter q of 10 to 50 are mainly dispersed.

Moreover, a plurality of particle size distribution peaks are possessedwithin the range of the size parameter, and a plurality of scatterers ofthe materials of different refractive indexes are dispersed in mixture.With the above arrangement, eye safety is secured by effectivelyresulting the speckles while solving the problems of the difficulties inmonodispersion at a high density, the nonuniformity of the near-fieldpattern and the skirt trailing of the far-field pattern. With regard tothe relation between the size parameter q and the refractive indexdifference Δn of the scatterers mainly dispersed as the base material ofthe first region, a criterion for selecting the desirable particlespecies can be obtained by sealing the size parameter q so that aproduct Δn·q falls within a range of approximately two to eight orparticularly proximate to three.

By carrying out the high density dispersion so that the average nearestneighbor distance of the scatterers in the first region becomesapproximately twenty times or less than the particle size mode Ds, adispersion condition in which the spatial coherency is reduced extremelyefficiently inside a minute volume on the millimeter order, can easilybe found. Moreover, frequency fluctuations induced by the return lightcan be generated in the semiconductor laser itself.

Moreover, by constituting the first region of a gel-like or rubber-likematerial (elastomer) as the base material, the first region can bestably maintained even through the process like the transfer moldingduring which a high pressure is applied while obtaining a uniformdispersion at a volume ratio of 0.5 vol % to 30 vol % or particularly ata scatterer density of 1 vol % to 10 vol %. Particularly, a silicone gelof which the viscosity before hardening is not greater than about 6000mPa·s is suitably used with a simple inexpensive dispersion and kneadingdevice. Furthermore, by dispersing the scatterers in the gel-like orrubber-like base material and providing the first region adjacent to thesemiconductor laser, a heat radiation property and a stress alleviatingeffect are obtained, and the coherent backscattering is generated,allowing the timewise coherency of the semiconductor laser to bereduced.

Moreover, the semiconductor laser is directly or indirectly fixed on thebottom surface of the recess portion constructed of the wall surface andthe bottom surface that have a metal layer at least part of theoutermost surfaces, and at least part of the wall surface and/or of thebottom surface that constitute the recess portion serves as a reflectivesurface to the scattered light in the first region and as a retentionsurface for defining the configuration of the first region. With thisarrangement, the dimensions of the multiple scattering region can beclearly defined, and the scatterer density can be controlled.Particularly, due to the arrangement that the metal layer on theoutermost surface constitutes the recess portion, the scattered lightgenerated in the first region that is stably formed and retained in therecess portion has its waves guided toward the second region as a whole.Particularly, by using silver for the metal layer on the outermostsurface, the reflectance at the interface that has the aforementionedwaveguiding effect can be improved, and the problem of the accumulationof photoabsorption peculiar to the multiple scattering can berestrained.

Moreover, the metal layer on the outermost surface is continuouslyformed so that the substances other than the metal are not exposed atleast in the principal portion within a range in which the scatteredlight spatially distributed in the first region reaches. With thisarrangement, the scattered light spatially distributed in the firstregion can be effectively prevented from leaking in the directions(toward the substrate side and so on) other than the optical axisdirection, and this allows the light source size to be definite withoutcausing a loss of the optical output and allows the spatial coherency tobe efficiently reduced.

Moreover, by making the surface of the metal layer formed in at leastpart of the wall surface of the recess portion serve as the reflectivesurface for changing the optical axis direction of the outgoing beam ofthe semiconductor laser toward the interface between the first regionand the second region, the scattering optical path length is effectivelyextended, and at least initial scattering (irregular reflection) isgenerated on the reflective surface obtained through the simple process,allowing a sufficient multiple scattering effect to be obtained even ina small-sized multiple scattering optical system.

In the above-mentioned construction, the size parameter q of the mainlydispersed scatterers should preferably satisfy the range ofapproximately 1 to 15 and particularly the range of 1 to 10. With thisarrangement, the ballistic straight light component is effectivelyattenuated, and the optical output that can be used as a light source isefficiently obtained. The refractive index difference Δn between thebase material and the scatterers of the first region is set not smallerthan 0.1 and particularly not smaller than 0.15, and this allows apreferable scattering characteristic to be obtained in the multiplescattering optical system.

Moreover, the aforementioned construction allows a satisfactory opticalcharacteristic free from loss of the optical output to be obtained byconstituting the eye-safe minute multiple scattering optical system mosteasily and alleviating the influence of the hiding power of thescatterers. Moreover, the end surface emitting type semiconductor lasercan be mounted in a simple die bonding form so that the optical axis ofthe outgoing beam of the semiconductor laser becomes approximatelyparallel to the bottom surface of the recess portion and manufactured atlow cost.

By mixing and dispersing the main scatterers that have a comparativelysmall particle size of a size parameter q of about 1 to 15 with thesubordinate scatterers that have a comparatively large particle size ofa size parameter q of not greater than about 50, a condition thatsatisfies eye safety while facilitating the dispersion process is found.Moreover, also by forming the first region into a laminate of theportion mainly constructed of the scatterers of a comparatively smallparticle size of a size parameter q of about 1 to 15 and the portionmainly constructed of the scatterers of a relatively large particle sizeidentical to or different from the above-mentioned scatterers, thesimplification of the dispersion process and the speckle reducing effectcan be made compatible.

Moreover, at least the principal part of the wall surface thatconstitutes the recess portion is served as the reflective surface thatconverts a plurality of times the optical axis of the output light ofthe semiconductor laser. Particularly, there is provided a constructionin which, when the principal portion of the wall surface is made to havea specified inclination angle and no scatterer is assumed to exist, thelight reaches the opening of the recess portion by a reflectionfrequency of about two times to five times. With this arrangement, evenin a multiple scattering optical system in which the thickness of thesecond region is thin in the optical axis direction, sufficient multiplescattering and light output efficiency can be made compatible, and thespeckle reduction can be achieved by remarkably improving the uniformityof the secondary planar light source.

Moreover, the recess portion has its diameter expanding toward theopening, and the aspect ratio r that is the depthwise ratio with respectto the diameter of the bottom surface of the recess portion and theangle θ [deg] between the normal line of the wall surface and theoptical axis of the semiconductor laser satisfy the relation:max{2r,3}≦θ≦20r.With this arrangement, the preferable optimum conditions concerning themultiple scattering for speckle reduction and the accompanying problemof the accumulation of photoabsorption by the metal layer can be found.

In the above-mentioned construction, the size parameter q of thescatterers mainly dispersed in the first region is set within the rangeof approximately 10 to 50 and particularly within the range of 15 to 40.With this arrangement, the near-field pattern can be made uniform moreeffectively, and the size of the apparent light source can easily beextended. Moreover, when the scatterers of a comparatively largeparticle size are mainly contained, limitations on the combination ofthe base material and the kneading device in the dispersion process isalleviated. Moreover, an eye-safe light source device can be constitutedwithout adding an extra cost factor by mounting the general end surfaceemitting type high-power semiconductor laser by simple die bonding.

Moreover, in the first region, the opening portion and the bottomsurface portion of the recess portion have almost the same cylindricalcross-sectional configuration, and the aspect ratio r of the recessportion and the angle θ made between the normal line of the wall surfaceof the recess portion and the optical axis of the semiconductor lasersatisfy the relation:a tan(r/5)≦θ≦a tan(r/2).The wall surface of the recess portion is provided inclined relativelyto the optical axis of the emission port of the semiconductor laser.With this arrangement, even in a multiple scattering optical system inwhich the magnifier of the second region has a small thickness in theoptical axis direction, sufficient multiple scattering and light outputefficiency can be made compatible, and the speckle reduction can beachieved with the uniformity of the secondary planar light sourceremarkably improved.

In the above-mentioned construction, the size parameter q of thescatterers mainly dispersed in the first region is set within the rangeof approximately 10 to 50 and particularly within the range of 15 to 40.With this arrangement, the near-field pattern can be made uniform moreeffectively, and the apparent light source size can easily be enlarged.

In the construction in which the combination of the multipath reflectionand the multiple scattering is used, it is proper to mix and dispersethe main scatterers of a comparatively low refractive index differenceand a large particle size (about q≧10) with the subordinate scatterersof a comparatively high refractive index difference and a small particlesize (about q≦10). This arrangement facilitates making compatible thespeckle reduction of the secondary planar light source and theuniforming of the light intensity distribution with the improvement inthe output efficiency of the optical output. Moreover, it is proper tolaminate the portion constructed of the main scatterers of acomparatively low refractive index difference (typically about Δn≦0.1)and a large particle size (about q≧10) and the portion constructed ofthe subordinate scatterers of a comparatively high refractive indexdifference (about Δn≧0.1) and a small particle size (about q≦10) for theformation of the first region. This arrangement facilitates makingcompatible the speckle reduction of the secondary planar light sourceand the uniforming of the light intensity distribution with theimprovement in the output efficiency of the optical output.

Moreover, by using the light source device employing a semiconductorlaser that has the active layer including the InGaAs layer on the GaAssubstrate and has an emission wavelength within the range of 880 nm to920 nm as a transmission means, it becomes possible to perform opticaltransmission between the device and the Si photodiode that serves as areception means having a peak sensitivity wavelength in the wavelengthband of 880 nm to 920 nm. Therefore, by employing this light sourcedevice, an optical communication module, which satisfies the Class 1 eyesafety and concurrently has the lowest cost and excellent electrical andoptical characteristics, can be constituted for wireless opticalcommunications.

Particularly, by making the other layers that have the active layerincluding the InGaAs quantum well layer on the GaAs substrate and arelatively high optical density of, for example, the quantum barrierlayer and the light guide layer located adjacent to the InGaAs layer andthe light guide layers provided besides the active layer and so on Alfree by constituting the layers including at least one of a ternarylayer and a quaternary layer expressed by In_(x)Ga_(1-x)As_(y)P_(1-y)(0≦x<1, 0<Y<1), it becomes possible to provide an eye-safe light sourcedevice that can produce the highest output in the wavelength band of 880nm to 920 nm. Therefore, by employing this light source device, anoptical communication module, which satisfies the Class 1 eye safety andconcurrently has the lowest cost and excellent electrical and opticalcharacteristics, can be constituted for wireless optical communications.

Moreover, by forming the layer that intentionally causes thefluctuations in the layer thickness during the crystal growth of theInGaAs quantum well layer, the quantum barrier layer, the light guidelayer, the light guide layer provided besides the active layer or thelike of the semiconductor laser, the spectral linewidth during the laseroperation can be extended. Moreover, by forming the layer thatintentionally causes local compositional fluctuations, the spectrallinewidth can be extended. By employing the semiconductor laser that hassuch spatial fluctuations in the composition or layer thickness, thespeckles can be further reduced by a synergistic effect with themultiple scattering optical system.

Particularly, when a semiconductor laser that includes a GaAsP ternarysystem material or an InGaAsP quaternary system material in its layerstructure is employed as the quantum well barrier layer and the lightguide layer located adjacent to the InGaAs active layer or the lightguide layer provided besides the active layer, the pseudo DFB(Distributed Feedback) type laser that has the pseudo grating of obscurephase in the cavity can be constituted by devising the growth condition.With this arrangement, the timewise coherency was able to be reduced byone or more orders of magnitude in comparison with an ordinaryhigh-power semiconductor laser. While extending the spectral linewidthas described above, sufficient reliability of high-power operation issecured. By modulating the injection current with a large amplitude, afurther effect of extending the spectral linewidth can be obtained.Therefore, a light source device that has a low operating currentcharacteristic on the highest level as a high-power semiconductor lasercan be constituted while obtaining the Class 1 eye safety with thespeckles almost completely lost.

Moreover, it is also preferable to include the AlGaAsSb layer that has adistortion as an electron blocking layer inside or in the vicinity ofthe active layer in the laser device that attaches importanceparticularly to the temperature characteristic than the high power. Itis needless to say that the various effects of the ternary or quaternarylayer expressed by InGaAs and In_(X)Ga_(1-X)As_(Y)P_(1-Y) can besimilarly obtained also in this case. Furthermore, by devising variouscrystal growth conditions of MOCVD, MBE (Molecular Beam Epitaxy) or thelike, the spatial fluctuations in the layer thickness and thecomposition can be generated also in the AlGaAsSb electron blockinglayer.

Moreover, by virtue of the arrangement that at least part of the wireconnected directly or indirectly to the semiconductor laser is locatedin the second region, the wire is peeled off together with the secondregion and disconnected to interrupt the electrification to thesemiconductor laser even if the second region is damaged or peeled offduring the electrification to the semiconductor laser. Therefore, thelaser beam of high coherency can be prevented from directly entering theuser's eyes, and this allows the safety to be further improved.

Moreover, in place of the mold type light source device on the resinsubstrate or the lead frame, this multiple scattering optical system isalso allowed to utilize a CAN package device, which can be utilized asan extremely small-sized eye-safe light source device. Particularly, byforming each of the first region and the second region into amulti-layer, various modification examples can be constituted.

Moreover, it is proper to constitute a light source device in which thesemiconductor laser is provided by a material system having an activelayer that oscillates in the ultraviolet region, and its output light issubjected to wavelength conversion into white light and multiplescattering particularly in the first region and emitted to the outside.Otherwise, it is proper to constitute a projection type display moduleincluding the aforementioned light source device. With this arrangement,a subminiature low-power-consumption projection module that projectsletter and image information on a wall surface or a paper surface can beprovided at extremely low cost.

In the aforementioned light source device, the first region of themultiple scattering optical system is formed as the minute regionlocated adjacent to the semiconductor laser, and therefore, thereception sensitivity is not deteriorated through integration andintegrated module formation with a photodiode. Accordingly, there can beprovided an unprecedented wireless optical communication module, whichconcurrently achieves the reduction in the size and cost on the samelevel as that of the existing IrDA transceiver and a high speed propertyand a wide communication area that is equivalent or superior to those ofthe existing optical wireless LAN products.

1. A light source device having a light source element from which output light is emitted to outside via a multiple scattering optical system, wherein the multiple scattering optical system includes at least a first region that is located adjacent to the light source element, and a second region that abuts on the first region and reaches the outside, of the first and second regions, at least the first region contains scatterers, and a density of the scatterers in the first region is higher than a density of scatterers in the second region, and the light source device has an amount of near-field pattern speckles σ_(PAR) that is within a range of: σ_(PAR)≧8×10⁻³.
 2. The light source device as claimed in claim 1, wherein the device comprises a recess portion having a wall surface and a bottom surface that define the first region, wherein a metal layer is formed on at least part of the wall surface and/or of the bottom surface, and the light source element is directly or indirectly fixed to the bottom surface, and a surface of the metal layer formed on the at least part of the wall surface and/or of the bottom surface of the recess portion serves as a reflective surface to scattered light of the output light from the light source element.
 3. The light source device as claimed in claim 2, wherein the metal layer on the at least part of the wall surface and/or of the bottom surface of the recess portion is continuously formed so that substances other than the metal are not exposed in a principal portion positioned within reach of the scattered light spatially distributed in the first region.
 4. The light source device as claimed in claim 2, wherein a surface of a metal layer formed on at least part of a wall surface of a recess portion serves as a reflective surface that changes an optical axis direction of an outgoing beam of the light source element toward an interface between the first and second regions, and assuming that a size parameter q, which represents a relation between a particle size mode Ds of the scatterers and a center wavelength λ in a base material of the first region of the light source element, is expressed by: q=(2π/λ)·(Ds/2), then the size parameter q of the first region falls within a range of approximately 1 to
 15. 5. The light source device as claimed in claim 2, wherein the surface of the metal layer formed on at least part of the wall surface of the recess portion serves as a reflective surface that changes an optical axis direction of an outgoing beam of the light source element a plurality of times, and assuming that a size parameter q, which represents a relation between a particle size mode Ds of the scatterers and a center wavelength λ in a base material of the first region of the light source element, is expressed by: q=(2π/λ)·(Ds/2), then the size parameter q of the first region falls within a range of approximately 10 to
 50. 6. The light source device as claimed in claim 5, wherein an opening of the recess portion has a diameter larger than that of the bottom surface, and assuming that a ratio of a depth to the diameter of the bottom surface of the recess portion is given as an aspect ratio, r, and an angle made between a normal line of the wall surface of the recess portion and the optical axis of the outgoing beam of the light source element is θ [deg], then a condition expressed by: max{2r, 3}≦θ≦20r is satisfied.
 7. The light source device as claimed in claim 5, wherein at least part of the wall surface of the recess portion forms a cylinder whose top and bottom have approximately same sectional configurations, and assuming that a ratio of a depth to a diameter of the cylinder of the recess portion is given as an aspect ratio, r, and an angle made between a normal line of the wall surface of the recess portion and the optical axis of the outgoing beam of the light source element is θ [deg], then a condition expressed by: max{a tan(r/5),3}≦θ≦a tan(r/2) is satisfied.
 8. The light source device as claimed in claim 1, wherein the second region has a lens portion.
 9. The light source device as claimed in claim 8, wherein the lens portion serves as a magnifier for at least a principal portion of a secondary planar light source formed at an interface between the first region and the second region.
 10. The light source device as claimed in claim 1, wherein, assuming that a size parameter q, which represents a relation between a particle size mode Ds of the scatterers and a center wavelength λ in a base material of the first region of the light source element, is expressed by: q=(2π/λ)·(Ds/2), then the particle size mode Ds of the scatterers is within a range that allows the size parameter q to fall within a range of approximately 1-50, and at least the first region includes a portion where the scatterers are dispersed at a high density so that an average nearest neighbor distance of the scatterers becomes equal to or smaller than twenty times the particle size mode Ds of the scatterers.
 11. The light source device as claimed in claim 1, wherein the first region employs a gel-like or rubber-like material as the base material.
 12. The light source device as claimed in claim 1, wherein the light source element is a semiconductor laser.
 13. The light source device as claimed in claim 12, wherein the semiconductor laser has an active layer including an InGaAs layer on a GaAs substrate and an emission wavelength within a range of from 880 nm to 920 nm inclusive.
 14. The light source device as claimed in claim 13, wherein the semiconductor laser has the active layer including the InGaAs layer on the GaAs substrate and includes at least one of a ternary layer or a quaternary layer which are expressed by In_(X)Ga_(1-X)As_(Y)P_(1-Y) (0≦X<1, 0<Y<1).
 15. The light source device as claimed in claim 12, wherein the semiconductor laser has spatial fluctuations in at least one of its composition or its layer thickness.
 16. The light source device as claimed in claim 15, wherein the semiconductor laser has the active layer including the InGaAs layer on the GaAs substrate and includes at least one of a ternary layer or a quaternary layer expressed by In_(X)Ga_(1-X)As_(Y)P_(1-Y) (0≦X<1, 0<Y<1) which has spatial fluctuations in its composition.
 17. The light source device as claimed in claim 1, wherein at least part of a wire connected directly or indirectly to the light source element exists inside the second region.
 18. An optical communication module employing the light source device claimed in claim 1 as a transmission means.
 19. The light source device as claimed in claim 1, wherein assuming that a transport mean free path of the scatterers is l_(AVE) and a dimension in the optical axis direction of the first region is L, then a transport optical depth L/l_(AVE) is approximately 1 to
 100. 20. The light source device as claimed in claim 1, wherein the amount of near-field pattern speckles σ_(PAR) is within a range expressed by: σ_(PAR)≦3×10⁻¹.
 21. The light source device as claimed in claim 1, wherein the light source element has an optical waveguide structure. 